Add PeerConnectionObserver#onRemoveTrack to android sdk

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

 /*
  *  Copyright 2015 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 package org.webrtc;
 
 /** Java wrapper for a C++ RtpReceiverInterface. */
 public class RtpReceiver {
   /** Java wrapper for a C++ RtpReceiverObserverInterface*/
   public static interface Observer {
     // Called when the first audio or video packet is received.
     public void onFirstPacketReceived(MediaStreamTrack.MediaType media_type);
   }
 
   final long nativeRtpReceiver;
   private long nativeObserver;
 
-  private MediaStreamTrack cachedTrack;
+  private final MediaStreamTrack cachedTrack;
 
   public RtpReceiver(long nativeRtpReceiver) {
     this.nativeRtpReceiver = nativeRtpReceiver;
+    // We can assume that an RtpReceiver always has an associated track.
     long track = nativeGetTrack(nativeRtpReceiver);
-    // We can assume that an RtpReceiver always has an associated track.
-    cachedTrack = new MediaStreamTrack(track);
+    String kind = MediaStreamTrack.nativeKind(track);
+    if (kind.equals("audio")) {
+      cachedTrack = new AudioTrack(track);
+    } else if (kind.equals("video")) {
+      cachedTrack = new VideoTrack(track);
+    } else {
+      throw new IllegalStateException("unexpected kind " + kind);
+    }
   }
 
   public MediaStreamTrack track() {
     return cachedTrack;
   }
 
   public boolean setParameters(RtpParameters parameters) {
     return nativeSetParameters(nativeRtpReceiver, parameters);
   }
 
@@ skipping 32 matching lines @@
   private static native RtpParameters nativeGetParameters(long nativeRtpReceiver);
 
   private static native String nativeId(long nativeRtpReceiver);
 
   private static native void free(long nativeRtpReceiver);
 
   private static native long nativeSetObserver(long nativeRtpReceiver, Observer observer);
 
   private static native long nativeUnsetObserver(long nativeRtpReceiver, long nativeObserver);
 };