AudioTrack是MediaStreamTrack的一个子类，负责音频的调节。
VideoTrack和Audio几乎完全一样，只是多了一个free的方法，然后添加的Renderer的类型不一样。

  //一个list里面存的是音频的渲染器
  private final LinkedList<AudioRenderer> renderers = new LinkedList<AudioRenderer>();

 //构造方法
 public AudioTrack(long nativeTrack) {
    super(nativeTrack);
  }

  //添加音频渲染器，与去除音频渲染器
  public void addRenderer(AudioRenderer renderer){
      renderers.add(renderer);
      nativeAddRenderer(nativeTrack, renderer.nativeAudioRenderer);
  }

  public void removeRenderer(AudioRenderer renderer){
      if(!renderers.remove(renderer)){
          return;
      }
      nativeRemoveRenderer(nativeTrack,renderer.nativeAudioRenderer);
      renderer.dispose();
  }

  private static native void nativeAddRenderer(long nativeTrack, long nativeRenderer);
  private static native void nativeRemoveRenderer(long nativeTrack, long nativeRenderer);

  //释放掉AudioTrack
  public void dispose(){
      while (!renderers.isEmpty()) {
          removeRenderer(renderers.getFirst());
      }
      super.dispose();
  }

  //方法含义同AudioTrack
  private final LinkedList<VideoRenderer> renderers = new LinkedList<VideoRenderer>();

  public VideoTrack(long nativeTrack) {
    super(nativeTrack);
  }

  public void addRenderer(VideoRenderer renderer) {
    renderers.add(renderer);
    nativeAddRenderer(nativeTrack, renderer.nativeVideoRenderer);
  }

  public void removeRenderer(VideoRenderer renderer) {
    if (!renderers.remove(renderer)) {
      return;
    }
    nativeRemoveRenderer(nativeTrack, renderer.nativeVideoRenderer);
    renderer.dispose();
  }

  public void dispose() {
    while (!renderers.isEmpty()) {
      removeRenderer(renderers.getFirst());
    }
    super.dispose();
  }

  private static native void free(long nativeTrack);

  private static native void nativeAddRenderer(long nativeTrack, long nativeRenderer);

  private static native void nativeRemoveRenderer(long nativeTrack, long nativeRenderer);