Make StubExoPlayer not abstract

All Player methods should throw
NotSupportedException.
Making it abstract prevents the compiler catching
non implemented methods.

PiperOrigin-RevId: 364974387

testutils/src/main/java/com/google/android/exoplayer2/testutil/StubExoPlayer.java

@@ -39,7 +39,7 @@ import java.util.List;
  * An abstract {@link ExoPlayer} implementation that throws {@link UnsupportedOperationException}
  * from every method.
  */
-public abstract class StubExoPlayer extends BasePlayer implements ExoPlayer {
+public class StubExoPlayer extends BasePlayer implements ExoPlayer {
 
   @Override
   public AudioComponent getAudioComponent() {