BQ: get rid of async in producer interface

- Get rid of the async flag in dequeueBuffer, allocateBuffers,
  waitForFreeSlotThenRelock, and QueueBufferInput.
- Instead use the persistent flags mDequeueBufferCannotBlock and
  mAsyncMode to determine whether to use the async behavior.

Bug 13174928

Change-Id: Ie6f7b9e46ee3844ee77b102003c84dddf1bcafdd