libstagefright: Handle FLAC clips with very small block sizes

- Clips with very small block sizes play with frequent glitches.
- As per the current design, the parser gives data in frames and
  as the size of each frame after decoding is very small, the framework
  does not receive PCM at the rate at which it expects resulting in
  glitches.
- Fix is to buffer data in the decoder and to read from parser only
  when needed.

CRs-Fixed: 676591
Change-Id: I0c79735ac16c0a9a821fbcadc172038a3145941a