Bug 4364249 Play position is 0 after flushing AudioTrack

AudioTrack::stop() is not synchronous, so a stop() followed
 by flush(), which is synchronous, will not always report
 a playhead position of 0 after being called.
This CL adds a flag to mark a track as flushed, and report the
 correct playhead position in this state.
Bug 5217011 has been created to address the real issue in the
 future, where flush could be made synchronous, to properly
 address bug 4364249.

Change-Id: Icf989d41a6bcd5985bb87764c287f3edb7e26d12