Nuplayer: During flush operation flush audiosink

Unrendered buffers written to sink are getting played in
case of seek during paused usecase, flush these buffers
to clear stale data corresponding to old position
(before seek) and since after flushing numframes
played gets reset, reset numframeswritten as well.

Change-Id: I1a4b2ae430bbe6959296e5a75005a91983c47b33