Fix a threading issue in AudioPlaybackHandler

        if (DBG) Log.d(TAG, "handleSynthesisDone()");

        final AudioTrack audioTrack = params.getAudioTrack();

        try {

            if (audioTrack != null) {

        if (audioTrack == null) {

            return;

        }

        if (DBG) Log.d(TAG, "Waiting for audio track to complete : " +

                audioTrack.hashCode());

        blockUntilDone(params);

        if (DBG) Log.d(TAG, "Releasing audio track [" + audioTrack.hashCode() + "]");

        // The last call to AudioTrack.write( ) will return only after

        // all data from the audioTrack has been sent to the mixer, so

                // it's safe to release at this point.

        // it's safe to release at this point. Make sure release() and the call

        // that set the audio track to null are performed atomically.

        synchronized (this) {

            // Never allow the audioTrack to be observed in a state where

            // it is released but non null. The only case this might happen

            // is in the various stopFoo methods that call AudioTrack#stop from

            // different threads, but they are synchronized on AudioPlayBackHandler#this

            // too.

            audioTrack.release();

            }

        } finally {

            params.setAudioTrack(null);

        }

        params.getDispatcher().dispatchUtteranceCompleted();

        mLastSynthesisRequest = null;

    }

    }

    private static void blockUntilDone(SynthesisMessageParams params) {

        if (params.mAudioTrack == null || params.mBytesWritten <= 0) {