Android: Add common function for adding/releasing native reference

webrtc/sdk/android/src/java/org/webrtc/WrappedNativeI420Buffer.java

 /*
  *  Copyright 2017 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 71 matching lines @@
   }
 
   @Override
   public VideoFrame.I420Buffer toI420() {
     retain();
     return this;
   }
 
   @Override
   public void retain() {
-    nativeAddRef(nativeBuffer);
+    JniCommon.nativeAddRef(nativeBuffer);
   }
 
   @Override
   public void release() {
-    nativeRelease(nativeBuffer);
+    JniCommon.nativeReleaseRef(nativeBuffer);
   }
 
   @Override
   public VideoFrame.Buffer cropAndScale(
       int cropX, int cropY, int cropWidth, int cropHeight, int scaleWidth, int scaleHeight) {
     return VideoFrame.cropAndScaleI420(
         this, cropX, cropY, cropWidth, cropHeight, scaleWidth, scaleHeight);
   }
-
-  private static native long nativeAddRef(long nativeBuffer);
-  private static native long nativeRelease(long nativeBuffer);
 }