Fix calculations for an obscure combo of MPEG audio options.

MPEGv2 and MPEGv2.5 Layer 2 audio payloads should 1152 samples per
access unit, not 576.  Adjust the frame size and samples out
calculations accordingly.

Also, adjust the max frame size in the MP3Extractor's MediaSource to
be closer to the theoretical worst case max frame size.  The
theoretical worst case for MPEG audio is 2881 bytes per frame, but the
max frame size being used was 32kB.  It has been changed to be 4kB in
order to remain a power of 2 allocation, but to be the power of 2
closest to the worst case.

Change-Id: If11f5a843b06e70151bbe8298cc54f954938d9d7