FFMPEG Plugin: Refactor start time handling in audio path.

- FFMPEG handles the relationship of start time between encoded and decoded
  audio data now by using the fTempPacket->dts and the
  fDecodedDataBuffer->pkt_dts fields. We still have to manually keep track of
  start times for consecutive audio frames though to support returning a number
  of audio frames that may assembled of partial AVFrames.

- The start time of the very first audio frame data packet returned by Decode()
  is now correctly calculated based on GetNextChunk() start times instead of
  being always zero.

- Introduce fRawDecodedAudio that serves as a container to store properties of
  the audio frames stored in fDecodedData. This prepares the population of the
  fHeader structure with audio frame properties needed to allow clients of
  BMediaDecoder::Decode() detect audio format changes in a later commit.

- Remove fStartTime as it is superflous now.