/**
     * 开启手机系统自带铃声
     */
    private void startAlarm() {
        mMediaPlayer = MediaPlayer.create(this, getSystemDefultRingtoneUri());
        mMediaPlayer.setLooping(true);
        try {
            mMediaPlayer.prepare();
        } catch (IllegalStateException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        mMediaPlayer.start();
    }

    private void stopAlarm(){
        mMediaPlayer.stop();
    }

    /**
     * 获取系统自带铃声的uri
     * @return RingtoneManager.getActualDefaultRingtoneUri(this,
            RingtoneManager.TYPE_RINGTONE)
     */
    private Uri getSystemDefultRingtoneUri() {
        return RingtoneManager.getActualDefaultRingtoneUri(this,
                RingtoneManager.TYPE_RINGTONE);
    }