Fix flush() followed by start()

        // track was already in the active list, not a problem

        if (status == ALREADY_EXISTS) {

            status = NO_ERROR;

        } else {

            // Acknowledge any pending flush(), so that subsequent new data isn't discarded.

            // It is usually unsafe to access the server proxy from a binder thread.

            // But in this case we know the mixer thread (whether normal mixer or fast mixer)

            // isn't looking at this track yet:  we still hold the normal mixer thread lock,

            // and for fast tracks the track is not yet in the fast mixer thread's active set.

            ServerProxy::Buffer buffer;

            buffer.mFrameCount = 1;

            (void) mAudioTrackServerProxy->obtainBuffer(&buffer);

        }

    } else {

        status = BAD_VALUE;