libmedia: avoid processing stale callback events when track recycled

when track is reused from SoundPool and started again, the
previous callback events could be processed immediately after
track started, the new playback will be stopped then

clear the stale cblk flags when recycled track is started again to
avoid triggering unexpected callback events

Change-Id: Ifc6fd973c655e9c7866391e7004016a7df9a9b9c
CRs-Fixed: 819973