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