Android EasyPlayer声音自动停止、恢复，一键静音等功能

int getPlayState ()
Returns the playback state of the AudioTrack instance.
获取当前的播放状态。这个接口会返回PLAYSTATE_STOPPED、PLAYSTATE_PAUSED、PLAYSTATE_PLAYING
三种状态，分别表示未播放、暂停中、正在播放

void pause ()
Pauses the playback of the audio data. Data that has not been played back will not be discarded. Subsequent calls to play() will play this data back. See flush() to discard this data.
暂停播放音频数据。已经在缓冲区中的未播放数据将不会被丢弃，在下次play的时候继续播放。调用flush则会丢弃缓冲数据。

void play ()
Starts playing an AudioTrack.
开始播放

int setStereoVolume (float leftGain, 
        float rightGain)

Sets the specified left and right output gain values on the AudioTrack.
设置左右声道的音量增益。

mAudioEnable = mAudioTrack!=null &&　mAudioTrack.getPlayState()==PLAYSTATE_PLAYING;

public void setAudioEnable(boolean enable) {
   mAudioEnable = enable;
   AudioTrack at = mAudioTrack;
   if (at != null) {
     synchronized (at) {
       if (!enable) {
         at.pause();
         at.flush();
       } else {
         at.flush();
         at.play();
       }
     }
   }
 }

int requestAudioFocus (AudioManager.OnAudioFocusChangeListener l, 
        int streamType, 
        int durationHint)
Request audio focus. Send a request to obtain the audio focus
请求获取音频焦点。
第一个参数为音频焦点更改时的回调；
第二个参数为音频类型，在我们调节音量时可以看到有若干种音量，就对应的这里的streamType，这里我们基本用MUSIC，表示“媒体”。
第三个参数表示获取焦点的“时长”，有如下几种情况：
AUDIOFOCUS_GAIN_TRANSIENT
表示仅仅为临时获取焦点。比如播放导航语音、通知声音等，属于时间很短暂的情况；
AUDIOFOCUS_GAIN_TRANSIENT_MAY_DUCK
表示为DUCK模式，表示当获取焦点后，允许先前获取过焦点的程序在降低输出音量的前提下继续播放。
AUDIOFOCUS_GAIN_TRANSIENT_EXCLUSIVE
痛第一种情况类似，只是不允许系统再播放其他声音。通常应用在语音备忘、语音识别等情况；
AUDIOFOCUS_GAIN
表示要获取焦点的时长未知。比如播放音乐等等。

当获取到焦点时，函数放回AUDIOFOCUS_REQUEST_GRANTED，当获取失败时，返回AUDIOFOCUS_REQUEST_FAILED

// 获取AudioManager实例
final AudioManager am = (AudioManager) mContext.getSystemService(Context.AUDIO_SERVICE);
AudioManager.OnAudioFocusChangeListener l = new AudioManager.OnAudioFocusChangeListener() {
  @Override
  public void onAudioFocusChange(int focusChange) {
    if (focusChange == AudioManager.AUDIOFOCUS_GAIN) {// 焦点获取到了，那继续播放，并恢复音量。
      AudioTrack audioTrack = mAudioTrack;
      if (audioTrack != null) {
        audioTrack.setStereoVolume(1.0f, 1.0f);
        if (audioTrack.getPlayState() == AudioTrack.PLAYSTATE_PAUSED) {
          audioTrack.flush();
          audioTrack.play();
        }
      }
    } else if (focusChange == AudioManager.AUDIOFOCUS_LOSS) {// 焦点丢失了，暂停播放。
       AudioTrack audioTrack = mAudioTrack;
      if (audioTrack != null) {
        if (audioTrack.getPlayState() == AudioTrack.PLAYSTATE_PLAYING) {
          audioTrack.pause();
        }
      }
    } else if (focusChange == AudioManager.AUDIOFOCUS_LOSS_TRANSIENT_CAN_DUCK) { // 焦点丢失了，但是允许在降低音量的前提下继续播放，那么降低声音。
      AudioTrack audioTrack = mAudioTrack;
      if (audioTrack != null) {
        audioTrack.setStereoVolume(0.5f, 0.5f);
      }
    }
  }
};
// 因为这里要获得的焦点无法预知时长，因此用AUDIOFOCUS_GAIN模式。
int requestCode = am.requestAudioFocus(l, AudioManager.STREAM_MUSIC, AudioManager.AUDIOFOCUS_GAIN);
if (requestCode == AudioManager.AUDIOFOCUS_REQUEST_GRANTED) {
  // 成功获取到了焦点。那启动播放
  AudioTrack audioTrack = mAudioTrack;
  if (audioTrack != null) {
    audioTrack.setStereoVolume(1.0f, 1.0f);
    if (audioTrack.getPlayState() == AudioTrack.PLAYSTATE_PAUSED) {
      audioTrack.flush();
      audioTrack.play();
    }
  }
}else{ // 没有获取到音频焦点。那不播放声音
  AudioTrack audioTrack = mAudioTrack;
  if (audioTrack != null) {
    if (audioTrack.getPlayState() == AudioTrack.PLAYSTATE_PLAYING) {
      audioTrack.pause();
    }
  }
}