Adding PeerConnection.Observer.onTrack to the Java SDK.

Bug: webrtc:8869
Change-Id: I4c33f9ddf293af8c093a8726431a3574ff2b6e39
Reviewed-on: https://webrtc-review.googlesource.com/73966
Commit-Queue: Seth Hampson <shampson@webrtc.org>
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23155}

sdk/android/api/org/webrtc/PeerConnection.java

@@ -109,6 +109,13 @@ public class PeerConnection {
      * setRemoteDescription.
      */
     @CalledByNative("Observer") void onAddTrack(RtpReceiver receiver, MediaStream[] mediaStreams);
+
+    /**
+     * Triggered when the signaling from SetRemoteDescription indicates that a transceiver
+     * will be receiving media from a remote endpoint. This is only called if UNIFIED_PLAN
+     * semantics are specified. The transceiver will be disposed automatically.
+     */
+    @CalledByNative("Observer") default void onTrack(RtpTransceiver transceiver){};
   }
 
   /** Java version of PeerConnectionInterface.IceServer. */