DefaultVideoSink: implement initialize() and isInitialized()

The implementation is straightforward as there is nothing to init.

PiperOrigin-RevId: 683596706

libraries/exoplayer/src/main/java/androidx/media3/exoplayer/video/DefaultVideoSink.java

@@ -67,12 +67,12 @@ import java.util.concurrent.Executor;
 
   @Override
   public void initialize(Format sourceFormat) {
-    throw new UnsupportedOperationException();
+    // Do nothing as there is no initialization needed.
   }
 
   @Override
   public boolean isInitialized() {
-    throw new UnsupportedOperationException();
+    return true;
   }
 
   @Override

libraries/exoplayer/src/main/java/androidx/media3/exoplayer/video/PlaybackVideoGraphWrapper.java

@@ -396,6 +396,7 @@ public final class PlaybackVideoGraphWrapper implements VideoSinkProvider, Video
     } catch (VideoFrameProcessingException e) {
       throw new VideoSink.VideoSinkException(e, sourceFormat);
     }
+    defaultVideoSink.initialize(sourceFormat);
     state = STATE_INITIALIZED;
     return videoGraph.getProcessor(/* inputIndex= */ 0);
   }