stagefright-plugins: Updates for FFMPEG 2.5

 * Update for API changes in FFMPEG 2.5
 * Clean up all the warnings
 * Remove homegrown timestamp handling, it's no longer needed
 * FFMPEG software video decoders perform quite well-
   We now have a formidable media stack that can play nearly
   anything!

Change-Id: I910e24a60f29968894b62d13b16c6b5c9b3b42a6