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
diff --git a/include/media/AudioTrack.h b/include/media/AudioTrack.h
index df30e8c..923518d 100644
--- a/include/media/AudioTrack.h
+++ b/include/media/AudioTrack.h
@@ -481,6 +481,7 @@
     bool                    mMarkerReached;
     uint32_t                mNewPosition;
     uint32_t                mUpdatePeriod;
+    bool                    mFlushed; // FIXME will be made obsolete by making flush() synchronous
     uint32_t                mFlags;
     int                     mSessionId;
     int                     mAuxEffectId;