Relanding: Break peerconnection_jni.cc into multiple files, in "pc" directory.

webrtc/sdk/android/src/jni/pc/audiotrack_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.
  */
-#include "webrtc/sdk/android/src/jni/audio_jni.h"
+
+#include "webrtc/api/mediastreaminterface.h"
+#include "webrtc/sdk/android/src/jni/jni_helpers.h"
 
 namespace webrtc_jni {
 
-rtc::scoped_refptr<webrtc::AudioDecoderFactory> CreateAudioDecoderFactory() {
-  return nullptr;
-}
-
-rtc::scoped_refptr<webrtc::AudioEncoderFactory> CreateAudioEncoderFactory() {
-  return nullptr;
+JOW(void, AudioTrack_nativeSetVolume)
+(JNIEnv*, jclass, jlong j_p, jdouble volume) {
+  rtc::scoped_refptr<webrtc::AudioSourceInterface> source(
+      reinterpret_cast<webrtc::AudioTrackInterface*>(j_p)->GetSource());
+  source->SetVolume(volume);
 }
 
 }  // namespace webrtc_jni