Improvements/fixes to ACodec.

- Make sure ACodec reverts its state when it's shutdown
- Defer "resume" to after handling the OutputPortSettingsChange

- If the OMX_EventPortSettingsChanged event comes in while we're flushing, defer it
  and make sure the output port can be disabled by deleting all buffers not already
  owned by the component.

Change-Id: I1f8cdffa71237b57d4275a48b834647a7b263e8b