audiodecoder: flushing without check the output samples

The sub-class flush operatrion will be skipped if no any output samples. It casue the sub-cass don't receive the flush command if the audio decode process is non-blocking in handle_frame.

In some platforms, the compress audio data pushed into DSP asynchronously, and don't get output sample immediately. In this case, the sub-class flush should be executed in order to clear any input buffer queue in DSP part before output any audio samples.