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/RtpTransceiver.java

@@ -196,6 +196,7 @@ public class RtpTransceiver {
     nativeStop(nativeRtpTransceiver);
   }
 
+  @CalledByNative
   public void dispose() {
     cachedSender.dispose();
     cachedReceiver.dispose();