Support building WebRTC without audio and video for Android

webrtc/sdk/android/src/jni/null_audio_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"
+
+namespace webrtc_jni {
+
+rtc::scoped_refptr<webrtc::AudioDecoderFactory> CreateAudioDecoderFactory() {
+  return rtc::scoped_refptr<webrtc::AudioDecoderFactory>();
+}
+
+rtc::scoped_refptr<webrtc::AudioEncoderFactory> CreateAudioEncoderFactory() {
+  return rtc::scoped_refptr<webrtc::AudioEncoderFactory>();
+}
+
+}  // namespace webrtc_jni