Merge "libmediaplayerservice: Explicitly force callbacks to stop running" into main

    {

        Mutex::Autolock lock(mLock);

        track = mTrack;

    }

    // do not hold lock while joining.

    if (track) {

        track->stopAndJoinCallbacks();

    }

    {

        Mutex::Autolock lock(mLock);

        close_l(); // clears mTrack

    }

    // destruction of the track occurs outside of mutex.