android-youtube-player, an open source alternative to the official YouTube Player API: https://medium.com/@soffritti.pierfrancesco/how-to-play-youtube-videos-in-your-android-app-c40427215230

layout.xml

<LinearLayout
  xmlns:android="http://schemas.android.com/apk/res/android"
  xmlns:app="http://schemas.android.com/apk/res-auto"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  android:orientation="vertical" >
  
<com.pierfrancescosoffritti.androidyoutubeplayer.core.player.views.YouTubePlayerView
  android:id="@+id/youtube_player_view"
  android:layout_width="match_parent"
  android:layout_height="wrap_content"
      
  app:videoId="S0Q4gqBUs7c"
  app:autoPlay="true"
  app:showFullScreenButton="false" />
  
</LinearLayout>