Only check the codec specific data when the output buffer contains kKeyIsCodecConfig in MP4 writer

o Assume there is only a single output buffer containing such information.
  This simplifies the logic in MP4 file writer

o Output SPS and PPS in the very first buffer for software AVC encoder
  This is to make AVC encoder work with the MP4 file writer

o Add timestamp value for codec config data

Change-Id: Iad27a04579e6028332429cd0bebd30976041e997