libstagefright: Fix to handle corrupted AAC clips

- AAC decoder was returning SUCCESS by default unless
  the state is changed to ERROR.
- For some cases, where the clip is corrupted, it will
  return SUCCESS even though it has not decoded anything.
- Fixed this by initializing the state to -1.

Change-Id: I802329ecf95a10911d8adecff57f0ea4a5a78294
CRs-Fixed: 284827, 285631