Change the background image when the audio file is running in the background in Android

When the audio file is running in the background, I try to change the image in the Android application, but there is a problem. Here is my code:

public class SongActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_song);
        final RelativeLayout rel = (RelativeLayout)findViewById(R.id.activity_song);
        MediaPlayer mp = MediaPlayer.create(SongActivity.this,R.raw.thesong);
        mp.start();
        rel.setBackgroundResource(R.drawable.bday);
        Runnable r = new Runnable() {
            @Override
            public void run() {
                synchronized (this){
                try {
                    wait(5000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                }
            }
        };
        Thread test = new Thread(r);
        test.start();
        rel.setBackgroundResource(R.drawable.bday2);
}
}

Please let me know if I can modify it or if it needs to be overhauled. Thank you in advance

resolvent:

Add ID

<?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="match_parent"
    android:orientation="vertical"
    android:id="@+id/linear_layout">

    </LinearLayout>

In code, just replace

final RelativeLayout rel = (RelativeLayout)findViewById(R.id.activity_song);

And

      final RelativeLayout rel = (RelativeLayout)findViewById(R.id.linear_layout);
      Resources res = getResources();
      Drawable drawable = res.getDrawable(R.drawable.image);
      rel.setBackground(drawable);