Add PeerConnectionObserver#onRemoveTrack to android sdk

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

Index: webrtc/sdk/android/api/org/webrtc/PeerConnection.java
diff --git a/webrtc/sdk/android/api/org/webrtc/PeerConnection.java b/webrtc/sdk/android/api/org/webrtc/PeerConnection.java
index b2155c42e4d0e79561adb746a3ac67a989fbc179..ab032ea004f4456f15494097128cf98b1822f913 100644
--- a/webrtc/sdk/android/api/org/webrtc/PeerConnection.java
+++ b/webrtc/sdk/android/api/org/webrtc/PeerConnection.java
@@ -92,6 +92,13 @@ public class PeerConnection {
      * 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. */