FFMPEG Plugin: Refactor out audio frames chunk decoding.

- Main purpose is to prepare auto detection of audio frame properties for
  media formats that encode those properties in the frames themself (e.g. MP3)
  instead of in the container format (e.g. WMA).
  The main difference between akin named methods _DecodeNextAudioFrame() and
  _DecodeNextAudioFrameChunk() is that the former method deals with providing
  the exact number of audio frames expected by the caller of
  BMediaDecoder::Decode() and the latter deals with decoding any number of
  audio frames at all.

- New documentation added and existing documentationupdated accordingly.

- No functional change intended.