FFMPEG Plugin: Refactor out actual audio decoding part.

- Main reason for this refactorin is to increase readability and thus make the
  audio deccode path more comprehensible.

- Added documentation for the new method accordingly.

- Small refactoring for detecting when to update fRawDecodedAudio's properties.
  This is a preparation step for factoring out the flushing of the
  fDecodedDataBuffer in a later commit.

- No functional change intended.