Prevent stopped audio tracks from mixing data

// ExtendedAudioBufferProvider interface

// Note that framesReady() takes a mutex on the control block using tryLock().

// This could result in priority inversion if framesReady() is called by the normal mixer,

// as the normal mixer thread runs at lower

// priority than the client's callback thread:  there is a short window within framesReady()

// during which the normal mixer could be preempted, and the client callback would block.

// Another problem can occur if framesReady() is called by the fast mixer:

// the tryLock() could block for up to 1 ms, and a sequence of these could delay fast mixer.

// FIXME Replace AudioTrackShared control block implementation by a non-blocking FIFO queue.

// framesReady() may return an approximation of the number of frames if called

// from a different thread than the one calling Proxy->obtainBuffer() and

// Proxy->releaseBuffer(). Also note there is no mutual exclusion in the

// AudioTrackServerProxy so be especially careful calling with FastTracks.

size_t AudioFlinger::PlaybackThread::Track::framesReady() const {

    if (mSharedBuffer != 0 && (isStopped() || isStopping())) {

        // Static tracks return zero frames immediately upon stopping (for FastTracks).

        // The remainder of the buffer is not drained.

        return 0;

    }

    return mAudioTrackServerProxy->framesReady();

}