Bug 5156756 Fix AAC ADTS header size computation

This previous frame size computation code had two incorrect
 assumptions:
 1/ the ADTS frame length value already contains the header size
 2/ the ADTS header is not of fixed size: it is 2 bytes bigger
  if it contains the CRC value.
For 1/, the code worked because when the header size was added
 to the frame size, the + operator is of stronger precedence
 than the ? operator, so the size added was always 0 (instead
 of ADTS_HEADER_LENGTH.
For 2/, the code worked as long as there was no CRC in the ADTS
 data.

The fix consists in:
 - documenting what the frame length computation code returns
 - fixing the frame length computation
 - when computing the frame length, also returning the header
  size so the correct data can be sent to the decoder.

Change-Id: I92df72a9e531f594f762e63d62f9dee7b0109904