AudioTrack: fix head position after restore

The head position transfered to the new track
by restoreTrack_l() must take into account the frames that
are dropped from the old track to avoid a non recoverable
offset in the playback head position returned to applications.

Bug: 11230062.
Change-Id: I51143a08b95e8f264ed709ae2054360315f2b8b1
diff --git a/include/private/media/AudioTrackShared.h b/include/private/media/AudioTrackShared.h
index fe258ad..395f164 100644
--- a/include/private/media/AudioTrackShared.h
+++ b/include/private/media/AudioTrackShared.h
@@ -256,6 +256,8 @@
         return mEpoch;
     }
 
+    size_t      getFramesFilled();
+
 private:
     size_t      mEpoch;
 };