FFmpegExtractor: ignore fake album art MJPEG streams

Allows flac/m4a to be correctly parsed by Android

Change-Id: I6738afa1bf84cd930bda3b179f324c73379cfaea
Add additional WAV types to Stagefright supported list

* Stagefright WAVExtractor supports U8, S16, S24 WAV encoding.
* Stagefright WAVExtractor also supports MS GSM.

Change-Id: Ic1595d3ac179e715821903b25bf1e814a4e5b31e