Add PeerConnectionObserver#onRemoveTrack to android sdk

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

 /*
  *  Copyright 2013 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.
  */
 
@@ skipping 74 matching lines @@
     public void onDataChannel(DataChannel dataChannel);
 
     /** Triggered when renegotiation is necessary. */
     public void onRenegotiationNeeded();
 
     /**
      * Triggered when a new track is signaled by the remote peer, as a result of
      * setRemoteDescription.
      */
     public void onAddTrack(RtpReceiver receiver, MediaStream[] mediaStreams);
+
+    /**
+     * Triggered when a previously added remote track is removed by the remote peer,
+     * as a result of setRemoteDescription.
+     */
+    public default void
+      onRemoveTrack(RtpReceiver receiver, MediaStream[] mediaStreams) {}
   }
 
   /** Java version of PeerConnectionInterface.IceServer. */
   public static class IceServer {
     // List of URIs associated with this server. Valid formats are described
     // in RFC7064 and RFC7065, and more may be added in the future. The "host"
     // part of the URI may contain either an IP address or a hostname.
     public final String uri;
     public final String username;
     public final String password;
@@ skipping 326 matching lines @@
   private native RtpSender nativeCreateSender(String kind, String stream_id);
 
   private native List<RtpSender> nativeGetSenders();
 
   private native List<RtpReceiver> nativeGetReceivers();
 
   private native boolean nativeStartRtcEventLog(int file_descriptor, int max_size_bytes);
 
   private native void nativeStopRtcEventLog();
 }