libmedia: correct latency computing of AudioRecord

- Originally, audio record latency is calculated by frame
  buffers allocated for this track, while the actual
  latency is determined by audio hal.
- Compute the track latency by frame count returned
  from audio hal when in TRANSFER_CALLBACK mode

Change-Id: I26e5e47e8cc3720895b962f7aab8a595a54b7c83