libstagefright: add timeout to avoid blocking readAt function call

- this patch is to fix the corner case in the patch "Fix ANR issue
  in HTTP Streaming". The related change Id is
  Ief280b7e23727606746e1ca2bf4caa4a3044b7bc. In this corner case,
  onFetch is blocked on fetching data from network, and onRead on
  the same thread has no chance to be called. As the result, readAt
  is always waiting for the notification in onRead. It caused to
  block the other related function calls in AwesomePlayer.
- add the timeout in readAt function
- add the generation ID for checking whether onRead runs by the
  outdated message

Change-Id: Idb8942c6cff245c63e826792fffb46586a401a70