Add PeerConnectionObserver#onRemoveTrack to android sdk

webrtc/examples/androidapp/src/org/appspot/apprtc/PeerConnectionClient.java

Index: webrtc/examples/androidapp/src/org/appspot/apprtc/PeerConnectionClient.java
diff --git a/webrtc/examples/androidapp/src/org/appspot/apprtc/PeerConnectionClient.java b/webrtc/examples/androidapp/src/org/appspot/apprtc/PeerConnectionClient.java
index 02dac1582add36db7ab255e3b8de221685f248ea..218281166b39c88b3e1e7cb71482acc00fe518dc 100644
--- a/webrtc/examples/androidapp/src/org/appspot/apprtc/PeerConnectionClient.java
+++ b/webrtc/examples/androidapp/src/org/appspot/apprtc/PeerConnectionClient.java
@@ -40,6 +40,7 @@ import org.webrtc.IceCandidate;
 import org.webrtc.Logging;
 import org.webrtc.MediaConstraints;
 import org.webrtc.MediaStream;
+import org.webrtc.MediaStreamTrack;
 import org.webrtc.PeerConnection;
 import org.webrtc.PeerConnection.IceConnectionState;
 import org.webrtc.PeerConnectionFactory;
@@ -1277,6 +1278,9 @@ public class PeerConnectionClient {
 
     @Override
     public void onAddTrack(final RtpReceiver receiver, final MediaStream[] mediaStreams) {}
+
+    @Override
+    public void onRemoveTrack(final RtpReceiver receiver, final MediaStream[] mediaStreams) {}
   }
 
   // Implementation detail: handle offer creation/signaling and answer setting,