Add @deprecated javadoc to StubExoPlayer#getPlaybackError

This should be inherited from Player#getPlaybackError, but lint still
complains and this seems like the easiest fix.

PiperOrigin-RevId: 342598368

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

@@ -97,6 +97,7 @@ public abstract class StubExoPlayer extends BasePlayer implements ExoPlayer {
     throw new UnsupportedOperationException();
   }
 
+  /** @deprecated Use {@link #getPlayerError()} instead. */
   @Deprecated
   @Override
   public ExoPlaybackException getPlaybackError() {