Android: Add common function for adding/releasing native reference

webrtc/sdk/android/src/jni/pc/dtmfsender_jni.cc

 /*
  *  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 31 matching lines @@
   return reinterpret_cast<webrtc::DtmfSenderInterface*>(j_dtmf_sender_pointer)
       ->duration();
 }
 
 JOW(jint, DtmfSender_nativeInterToneGap)
 (JNIEnv* jni, jclass, jlong j_dtmf_sender_pointer) {
   return reinterpret_cast<webrtc::DtmfSenderInterface*>(j_dtmf_sender_pointer)
       ->inter_tone_gap();
 }
 
-JOW(void, DtmfSender_free)
-(JNIEnv* jni, jclass, jlong j_dtmf_sender_pointer) {
-  reinterpret_cast<webrtc::DtmfSenderInterface*>(j_dtmf_sender_pointer)
-      ->Release();
-}
-
 }  // namespace webrtc_jni