1º Passo: Crie um arquivo XML que representará sua nova actionbar. 2º Passo: Em MainActivity, troque a actionbar atual para a nova (já centralizada no layout)

action_bar_default.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_gravity="center"
    android:orientation="vertical">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:text="YOUR ACTIVITY TITLE"
        android:textColor="#ffffff"
        android:textSize="24sp" />
</LinearLayout>

MainActivity.java

import android.os.Bundle;
import android.support.v7.app.ActionBar;
import android.support.v7.app.AppCompatActivity;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // Essas 2 linhas é que fazem a mágica!
        getSupportActionBar().setDisplayOptions(ActionBar.DISPLAY_SHOW_CUSTOM);
        getSupportActionBar().setCustomView(R.layout.action_bar_default);
    }
}