FFMPEG Plugin: Fill out more media header fields in audio path.

- Some small refactoring when resetting fRawDecodedAudio. Instead of letting
  FFMPEG reset fRawDecodedAudio we do it manually to preserve the allocated
  memory in fRawDecodedAudio->opaque (otherwise FFMPEG's
  avcodec_get_frame_defaults() would NULLify the opaque pointer without
  releasing the allocated memory.

- Keep track of the total size of fDecodedData in fRawDecodedAudio->linesize[0]
  instead of relying on calculating it every time it is needed. This makes the
  code more comprehensible.