libmediaplayerservice: Avoid writing silence for offload tracks

-Audio Sink writes silence to the track if size returned from AudioPlayer
 is zero and when there is no subsequent output.
-Avoid writing silence for offload tracks as decoders do not expected
 any raw data.

CRs-Fixed: 615468
Change-Id: I5044d514126c413f56971e80815f232298f9685c