Media player service using : https://www.sitepoint.com/a-step-by-step-guide-to-building-an-android-audio-player-app/

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
          package="com.example.mediaplayer">

    <uses-permission android:name="android.permission.INTERNET"/>

    <permission android:name="android.permission.MEDIA_CONTENT_CONTROL"/>

    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />



    <uses-permission android:name="android.permission.READ_PHONE_STATE"/>

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN"/>

                <category android:name="android.intent.category.LAUNCHER"/>
            </intent-filter>
        </activity>

        <service
            android:name=".MediaPlayerService"
            android:enabled="true"
            android:exported="true">
        </service>
    </application>

</manifest>

Audio.java

package com.example.mediaplayer;

import java.io.Serializable;

public class Audio implements Serializable
{
    
    

    private String data;
	private String title;
    private String album;
    private String artist;

    public Audio(String data, String title, String album, String artist) {
        this.data = data;
        this.title = title;
        this.album = album;
        this.artist = artist;
    }

    public String getData() {
        return data;
    }

    public void setData(String data) {
        this.data = data;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public String getAlbum() {
        return album;
    }

    public void setAlbum(String album) {
        this.album = album;
    }

    public String getArtist() {
        return artist;
    }

    public void setArtist(String artist) {
        this.artist = artist;
    }
    
    
    
    
}

MainActivity.java

package com.example.mediaplayer;

import android.content.ComponentName;
import android.content.ContentResolver;
import android.content.Context;
import android.content.Intent;
import android.content.ServiceConnection;
import android.database.Cursor;
import android.net.Uri;
import android.os.Bundle;
import android.os.IBinder;
import android.provider.MediaStore;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;

import java.util.ArrayList;

public class MainActivity extends AppCompatActivity
{
	
	
	Button play, pause, stop, toggle;
	
	ArrayList<Audio> audioList;
	
	
	private MediaPlayerService player;
	boolean serviceBound = false;
	
	
	@Override
	protected void onCreate( Bundle savedInstanceState )
	{
		super.onCreate( savedInstanceState );
		setContentView( R.layout.activity_main );
		
		
		play= (Button) findViewById( R.id.play );
		pause= (Button) findViewById( R.id.pause );
		stop=(Button) findViewById( R.id.stop );
		toggle= (Button) findViewById( R.id.toggle );
		
		
		play.setOnClickListener( new View.OnClickListener()
		{
			@Override
			public void onClick( View view )
			{
			
				
				
				playAudio( R.raw.shape_of_you );
				
				
			}
		} );
		
		pause.setOnClickListener( new View.OnClickListener()
		{
			@Override
			public void onClick( View view )
			{
				if(serviceBound && player.isPlaying() && !player.isStopped())
					player.pauseMedia();
				
			}
		} );
		
		
		stop.setOnClickListener( new View.OnClickListener()
		{
			@Override
			public void onClick( View view )
			{
				if(serviceBound &&  player.isPlaying() && !player.isStopped() )
					player.stopMedia();
				
			}
		} );
		
		
		toggle.setOnClickListener( new View.OnClickListener()
		{
			@Override
			public void onClick( View view )
			{
				player.setRepeating( !player.isRepeating() );
				
			}
		} );
		
		
		
		
		
		
		
		
		
		
		/*loadAudio(this);
		//play the first audio in the ArrayList
		playAudio(audioList.get(0).getData());*/
		
	}
	
	
	
	//Binding this Client to the AudioPlayer Service
	private ServiceConnection serviceConnection = new ServiceConnection() {
		@Override
		public void onServiceConnected(ComponentName name, IBinder service) {
			// We've bound to LocalService, cast the IBinder and get LocalService instance
			MediaPlayerService.LocalBinder binder = (MediaPlayerService.LocalBinder) service;
			player = binder.getService();
			serviceBound = true;
			
			Toast.makeText(MainActivity.this, "Service Bound", Toast.LENGTH_SHORT).show();
		}
		
		@Override
		public void onServiceDisconnected(ComponentName name) {
			serviceBound = false;
		}
	};
	
	
	private void playAudio(int media) {
		//Check is service is active
		if (!serviceBound) {
			Intent playerIntent = new Intent(this, MediaPlayerService.class);
			playerIntent.putExtra("media", media);
			playerIntent.putExtra( "play",true );
			startService(playerIntent);
			bindService(playerIntent, serviceConnection, Context.BIND_AUTO_CREATE);
		} else {
			
			//Service is active
			//Send media with BroadcastReceiver
			if(!player.isStopped())
				player.playMedia();
			
			else
			{
				
				Intent playerIntent = new Intent(this, MediaPlayerService.class);
				playerIntent.putExtra("media", media);
				playerIntent.putExtra( "play",true );
				startService(playerIntent);
				bindService(playerIntent, serviceConnection, Context.BIND_AUTO_CREATE);
				
				
			}
		}
	}
	
	
	@Override
	public void onSaveInstanceState(Bundle savedInstanceState) {
		savedInstanceState.putBoolean("ServiceState", serviceBound);
		super.onSaveInstanceState(savedInstanceState);
	}
	
	@Override
	public void onRestoreInstanceState(Bundle savedInstanceState) {
		super.onRestoreInstanceState(savedInstanceState);
		serviceBound = savedInstanceState.getBoolean("ServiceState");
	}
	
	@Override
	protected void onDestroy()
	{
		super.onDestroy();
		if ( serviceBound )
		{
			unbindService( serviceConnection );
			//service is active
			player.stopSelf();
		}
		
	}
	
	
	
	
	
	
	private void loadAudio(Context context) {
		ContentResolver contentResolver = context.getContentResolver();
		
		Uri uri = MediaStore.Audio.Media.EXTERNAL_CONTENT_URI;
		String selection = MediaStore.Audio.Media.IS_MUSIC + "!= 0";
		String sortOrder = MediaStore.Audio.Media.TITLE + " ASC";
		Cursor cursor = contentResolver.query(uri, null, selection, null, sortOrder);
		
		if (cursor != null && cursor.getCount() > 0) {
			audioList = new ArrayList<>();
			while (cursor.moveToNext()) {
				String data = cursor.getString(cursor.getColumnIndex(MediaStore.Audio.Media.DATA));
				String title = cursor.getString(cursor.getColumnIndex(MediaStore.Audio.Media.TITLE));
				String album = cursor.getString(cursor.getColumnIndex(MediaStore.Audio.Media.ALBUM));
				String artist = cursor.getString(cursor.getColumnIndex(MediaStore.Audio.Media.ARTIST));
				
				// Save to audioList
				audioList.add(new Audio(data, title, album, artist));
			}
		}
		cursor.close();
	}
}

MediaPlayerService.java

package com.example.mediaplayer;

import android.app.Service;
import android.content.Context;
import android.content.Intent;
import android.content.res.AssetFileDescriptor;
import android.media.AudioManager;
import android.media.MediaPlayer;
import android.os.Binder;
import android.os.IBinder;
import android.telephony.PhoneStateListener;
import android.telephony.TelephonyManager;
import android.util.Log;

import java.io.IOException;

public class MediaPlayerService extends Service implements MediaPlayer.OnCompletionListener,
														   MediaPlayer.OnPreparedListener, MediaPlayer.OnErrorListener, MediaPlayer.OnSeekCompleteListener,
														   MediaPlayer.OnInfoListener, MediaPlayer.OnBufferingUpdateListener,

														   AudioManager.OnAudioFocusChangeListener
{
	
	
	// Binder given to clients
	private final IBinder iBinder = new LocalBinder();
	private AudioManager audioManager;
	private MediaPlayer mediaPlayer;
	//path to the audio file
	private int mediaFile;
	private int resumePosition;
	//Handle incoming phone calls
	private boolean ongoingCall = false;
	private PhoneStateListener phoneStateListener;
	private TelephonyManager telephonyManager;
	private boolean repeating = false;
	private boolean playing = false;
	private boolean paused = false;
	private boolean stopped = false;
	private boolean preparing = false;
	private boolean playOnInit = false;
	
	boolean wasPlaying =false;
	
	boolean FOCUS=false;
	
	@Override
	public int onStartCommand( Intent intent, int flags, int startId )
	{
		try
		{
			//An audio file is passed to the service through putExtra();
			mediaFile = intent.getExtras().getInt( "media" );
			playOnInit = intent.getExtras().getBoolean( "play" );
		}
		catch ( NullPointerException e )
		{
			stopSelf();
		}
		
		//Request audio focus
		if ( requestAudioFocus() == false )
		{
			//Could not gain focus
			stopSelf();
		}
		
		if ( mediaFile != -1 && mediaFile != 0 )
		{
			initMediaPlayer();
		}
		
		return super.onStartCommand( intent, flags, startId );
	}
	
	@Override
	public void onDestroy()
	{
		super.onDestroy();
		if ( mediaPlayer != null )
		{
			stopMedia();
			mediaPlayer.release();
		}
		removeAudioFocus();
	}
	
	@Override
	public IBinder onBind( Intent intent )
	{
		return iBinder;
	}
	
	private void initMediaPlayer()
	{
		mediaPlayer = new MediaPlayer();
		//Set up MediaPlayer event listeners
		mediaPlayer.setOnCompletionListener( this );
		mediaPlayer.setOnErrorListener( this );
		mediaPlayer.setOnPreparedListener( this );
		mediaPlayer.setOnBufferingUpdateListener( this );
		mediaPlayer.setOnSeekCompleteListener( this );
		mediaPlayer.setOnInfoListener( this );
		//Reset so that the MediaPlayer is not pointing to another data source
		mediaPlayer.reset();
		mediaPlayer.setLooping( isRepeating() );
		mediaPlayer.setAudioStreamType( AudioManager.STREAM_MUSIC );
		
		// Set the data source to the mediaFile location
		AssetFileDescriptor afd = getResources().openRawResourceFd(mediaFile);
		try
		{
			mediaPlayer.setDataSource(afd.getFileDescriptor(), afd.getStartOffset(), afd.getLength());
		}
		catch ( IOException e )
		{
			e.printStackTrace();
			stopSelf();
		}
		
		
		mediaPlayer.prepareAsync();
		preparing = true;
		
		
	}
	
	@Override
	public void onBufferingUpdate( MediaPlayer mp, int percent )
	{
		//Invoked indicating buffering status of
		//a media resource being streamed over the network.
	}
	
	@Override
	public void onCompletion( MediaPlayer mp )
	{
		//Invoked when playback of a media source has completed.
		
		stopMedia();
		
		playing = false;
		paused = false;
		stopped = true;
		
		
	}
	
	//Handle errors
	@Override
	public boolean onError( MediaPlayer mp, int what, int extra )
	{
		//Invoked when there has been an error during an asynchronous operation
		switch ( what )
		{
			case MediaPlayer.MEDIA_ERROR_NOT_VALID_FOR_PROGRESSIVE_PLAYBACK:
				Log.d( "MediaPlayer Error", "MEDIA ERROR NOT VALID FOR PROGRESSIVE PLAYBACK " + extra );
				break;
			case MediaPlayer.MEDIA_ERROR_SERVER_DIED:
				Log.d( "MediaPlayer Error", "MEDIA ERROR SERVER DIED " + extra );
				break;
			case MediaPlayer.MEDIA_ERROR_UNKNOWN:
				Log.d( "MediaPlayer Error", "MEDIA ERROR UNKNOWN " + extra );
				break;
		}
		return false;
	}
	
	@Override
	public void onPrepared( MediaPlayer mp )
	{
		//Invoked when the media source is ready for playback.
		
		preparing = false;
		
		if ( playOnInit )
		{
			playMedia();
			
			playOnInit = !playOnInit;
		}
		
		callStateListener();
	}
	
	@Override
	public boolean onInfo( MediaPlayer mp, int what, int extra )
	{
		//Invoked to communicate some info.
		return false;
	}
	
	@Override
	public void onSeekComplete( MediaPlayer mp )
	{
		//Invoked indicating the completion of a seek operation.
	}
	
	@Override
	public void onAudioFocusChange( int focusState )
	{
		//Invoked when the audio focus of the system is updated.
		switch ( focusState )
		{
			case AudioManager.AUDIOFOCUS_GAIN:
				// resume playback
				if ( mediaPlayer == null )
				{
					
				}
				else if ( wasPlaying )
				{
					
					wasPlaying =false;
					if(paused)
					playMedia();
				}
				mediaPlayer.setVolume( 1.0f, 1.0f );
				break;
			case AudioManager.AUDIOFOCUS_LOSS:
				// Lost focus for an unbounded amount of time: stop playback and release media player
				if ( mediaPlayer.isPlaying() )
				{
					mediaPlayer.stop();
				}
				mediaPlayer.release();
				mediaPlayer = null;
				break;
			case AudioManager.AUDIOFOCUS_LOSS_TRANSIENT:
				// Lost focus for a short time, but we have to stop
				// playback. We don't release the media player because playback
				// is likely to resume
				if ( mediaPlayer.isPlaying() )
				{
					mediaPlayer.pause();
					wasPlaying =true;
				}
				break;
			case AudioManager.AUDIOFOCUS_LOSS_TRANSIENT_CAN_DUCK:
				// Lost focus for a short time, but it's ok to keep playing
				// at an attenuated level
				if ( mediaPlayer.isPlaying() )
				{
					mediaPlayer.setVolume( 0.1f, 0.1f );
				}
				break;
		}
	}
	
	private boolean requestAudioFocus()
	{
		audioManager = (AudioManager) getSystemService( Context.AUDIO_SERVICE );
		int result = audioManager.requestAudioFocus( this, AudioManager.STREAM_MUSIC, AudioManager.AUDIOFOCUS_GAIN );
		if ( result == AudioManager.AUDIOFOCUS_REQUEST_GRANTED )
		{
			//Focus gained
			return true;
		}
		//Could not gain focus
		return false;
	}
	
	private boolean removeAudioFocus()
	{
		return AudioManager.AUDIOFOCUS_REQUEST_GRANTED ==
				audioManager.abandonAudioFocus( this );
	}
	
	public boolean isRepeating()
	{
		return repeating;
	}
	
	public void setRepeating( boolean repeating )
	{
		this.repeating = repeating;
		
		if ( mediaPlayer != null )
		{
			mediaPlayer.setLooping( repeating );
		}
	}
	
	public void playMedia()
	{
		if ( !mediaPlayer.isPlaying() )
		{
			mediaPlayer.start();
			playing = true;
			paused = false;
			stopped = false;
			wasPlaying =false;
		}
	}
	
	public void stopMedia()
	{
		if ( mediaPlayer == null )
		{
			return;
		}
		if ( mediaPlayer.isPlaying() )
		{
			mediaPlayer.stop();
			
		}
		
		playing = false;
		paused = false;
		stopped = true;
		
	}
	
	public void pauseMedia()
	{
		if ( mediaPlayer.isPlaying() )
		{
			mediaPlayer.pause();
			resumePosition = mediaPlayer.getCurrentPosition();
			
			playing = false;
			paused = true;
			stopped = false;
			
		}
	}
	
	public void resumeMedia()
	{
		if ( !mediaPlayer.isPlaying() )
		{
			mediaPlayer.seekTo( resumePosition );
			mediaPlayer.start();
			
			playing = true;
			paused = false;
			stopped = false;
			
		}
	}
	
	//Handle incoming phone calls
	private void callStateListener()
	{
		// Get the telephony manager
		telephonyManager = (TelephonyManager) getSystemService( Context.TELEPHONY_SERVICE );
		//Starting listening for PhoneState changes
		phoneStateListener = new PhoneStateListener()
		{
			@Override
			public void onCallStateChanged( int state, String incomingNumber )
			{
				switch ( state )
				{
					//if at least one call exists or the phone is ringing
					//pause the MediaPlayer
					case TelephonyManager.CALL_STATE_OFFHOOK:
					case TelephonyManager.CALL_STATE_RINGING:
						if ( mediaPlayer != null )
						{
							pauseMedia();
							ongoingCall = true;
						}
						break;
					case TelephonyManager.CALL_STATE_IDLE:
						// Phone idle. Start playing.
						if ( mediaPlayer != null )
						{
							if ( ongoingCall )
							{
								ongoingCall = false;
								resumeMedia();
							}
						}
						break;
				}
			}
		};
		// Register the listener with the telephony manager
		// Listen for changes to the device call state.
		telephonyManager.listen( phoneStateListener,
				PhoneStateListener.LISTEN_CALL_STATE );
	}
	
	public boolean isPlaying()
	{
		return playing;
	}
	
	
	public boolean isPaused()
	{
		return paused;
	}
	
	
	public boolean isStopped()
	{
		return stopped;
	}
	
	
	public boolean isPreparing()
	{
		return preparing;
	}
	
	
	public class LocalBinder extends Binder
	{
		public MediaPlayerService getService()
		{
			return MediaPlayerService.this;
		}
	}
	
	
}

Utils.java

package com.example.mediaplayer;

import android.content.Context;

/**
 * Created by tzia on 06-Jul-17.
 */

public class Utils
{
	
	public static int getFileFromAssetFolder(String fileName, Context context){
		
		
		return  context.getResources().getIdentifier( fileName, "raw", context.getPackageName() );
				
		
		
	}
	
}