ALAudioSink: Fix stop and flush of OpenAL source

- stopImpl() shall always issue alSourceStop(..) if state is not STOPPED
- Remove 'flush' hint for dequeueBuffer(..), we perform proper flush in respective method, see below
- flush() needs to issue:
   - stopImpl() - which should already dequeue all buffers
   - Explicitly dequeue all buffers: via 'alSourcei(alSource[0], AL.AL_BUFFER, 0)'
   - Then dequeue manually processed buffers: dequeueBuffer( false /* wait */ );
   - And dequeue _all_ buffers: dequeueForceAll();