GenericSource: refactor track selection for dedicated looper

 - move getFormatMeta, getSelectedTrack, selectTrack, and seekTo to
   dedicated looper
 - start & dequeueAccessUnit posts readBuffer requests instead of reading
   directly
 - call internal doGetFormatMeta in onPrepareAsync

Bug: 16892748
Change-Id: I5d5e34381f1dfceca655c52a201627b53454ad22
2 files changed