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
stagefright-plugins: Enable multithreaded decoding

 * Set thread_count to zero in order for FFMPEG to choose an appropriate
   number of threads based on available CPUs.

Change-Id: Ifa74195cc07c64f535fad132b5b56db3f611d461