Allow MediaSource instances to be reused

[stoleruedgar]

Issue description

Passing the same DynamicConcatenatingMediaSource to a new SimpleExoPlayer instance doubles the internal mediaSourceHolders which in turn breaks the Player behaviour

Reproduction steps

public class TestActivity extends Activity {

	private DynamicConcatenatingMediaSource playlist = null
	private SimpleExoPlayer player = null

	protected onCreate(Bundle savedInstance) {
		super.onCreate(savedInstance)
		playlist = ....init with mediasources       
	}	

	protected onStart() {
		super.onStart()      
		if (player == null) {
			player = ...init player
			if (playlist != null) {
				player.prepare(playlist, false, true)
			}
		} 
	}

	protected onStop() {
		super.onStop()
		player = ...release player
	}	
}

Put the activity in background and then bring the app in the foreground.

Version of ExoPlayer being used

r2.5.4

Device(s) and version(s) of Android being used

Samsung Galaxy S7, Android 7.0

[tonihei]

Media sources shouldn't be reused (documented here). This includes the dynamic concatenating media source as well as the single playlist item media sources.

[ojw28]

Could/should we add some assertions that check this and give a developer friendly error message? Seems like an easy mistake to make.

[tonihei]

I think we can also make DynamicConcatenatingMediaSource reusable (despite the general guideline). This would deliberately allow reusing the wrapping dynamic media source. Marking this as enhancement to do this.

[tonihei]

To clarify: this enhancement tracks reusing media sources in general. Reusing just the DynamicConcatenatingMediaSource wouldn't make sense if the contained media sources can't be reused.