stagefright-plugins: Implement frame dropping and AV sync

 * Keep the video from lagging behind the audio by dropping
   frames when we're too far behind.
 * Set options to improve video playback performance
 * It's still a quite terrible experience without hardware decoding,
   but prior to this patch it was possible that the first slow
   frame would be droped by Stagefright and the lag would just grow,
   resulting in a black screen.

Change-Id: Ib10933d385d593d8e887bca033f22b68a812bc7c
stagefright-plugins: Return target format in PCM params query

 * If the framework told us to do something else, we should do it.

Change-Id: Ie7777346012a96392a3baa9cca38c3ac31288b5d
stagefright-plugins: Use new OMX type for AC3

 * This is new in Lollipop. Google didn't implement a decoder though,
   so we can handle it.

Change-Id: I755e7f02de38053770bb53acae2a9a41549dfbad