libstagefright: Fix incorrect progress bar display during device switch

 -During offload to nonoffload playback switch (speakr to a2dp)
  audiotear down happens which reset current postion and duration.
 -do not reset the file duration during audio tear down and return
  the cached current position if audioplayer is not created.

Change-Id: I924d270d57aee1a9e79cea1c33bbd6bb731b201d