Android: Add common function for adding/releasing native reference

webrtc/sdk/android/api/org/webrtc/MediaSource.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.
  */
 
 package org.webrtc;
 
 /** Java wrapper for a C++ MediaSourceInterface. */
 public class MediaSource {
   /** Tracks MediaSourceInterface.SourceState */
   public enum State { INITIALIZING, LIVE, ENDED, MUTED }
 
   final long nativeSource; // Package-protected for PeerConnectionFactory.
 
   public MediaSource(long nativeSource) {
     this.nativeSource = nativeSource;
   }
 
   public State state() {
     return nativeState(nativeSource);
   }
 
   public void dispose() {
-    free(nativeSource);
+    JniCommon.nativeReleaseRef(nativeSource);
   }
 
   private static native State nativeState(long pointer);
-
-  private static native void free(long nativeSource);
 }