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.
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.
FFMPEG Plugin: Automatic detection of audio decoding parameters.

- Kudos to Marcus Overhagen for laying out the general idea of automatic
  detection by sharing some of his dvb code examples with me.
- Automatically detect the audio frame rate, channel count and sample format.
- Share audio sample format conversion code between AVFormatReader and
  AVCodecDecoder.
- Tested with several video and audio files via MediaPlayer.
- Tested also with test case mp3_decoder_test -after- removing the hard coded
  audio decoding parameters. Although the test shows that auto detection is
  working (via stepping through the auto detection code path) the complete test
  is still failing, due to missing implementation of incomplete audio frame
  decoding.
- Add and update the documentation accordingly.