libstagefright: Aggregate buffers in audio source

 -AudioSource currently receives callback whenever
  Audiorecord callback has data. The callbacks are
  with very small buffer sizes. These small buffer
  sizes lead to increase in number of messages that
  have to be passed between components. This leads
  to increase in power especially in camcorder
  use cases.
- To solve this, camcorder use cases configure
  AudioRecord in TRANSFER_SYNC mode so that
  buffers are aggregated inside AudioRecord.
  Soundrecorder, VOIP use cases are left untouched
  and continue to use TRANSFER_CALLBACK
Change-Id: Ie79028357656623e241c11bd017c69753f25d964