stagefright: Improve 24-bit support

 * Cleanup and fixes to allow FFMPEG codecs to output raw 24-bit PCM

Change-Id: Ideeb7cd7c6340f8054f99272796032939b561f81

Conflicts:
	media/libstagefright/Utils.cpp

	modified:   include/media/stagefright/MetaData.h
	modified:   media/libstagefright/AwesomePlayer.cpp
	modified:   media/libstagefright/OMXCodec.cpp
	modified:   media/libstagefright/Utils.cpp