Let the user enable MPEG2 hardware decoding

MPEG2 decoding has been disabled because mediacodec does not provide the right aspect ratio information after decoding the frames. As a result, MPEG2 files are played in software decoder, so it can’t keep up for 4K content.

Recommendation: create a user setting to force-enable mpeg2 hw decode path, and let user deal with any aspect ratio stretch with the video size option.

Internal BugID: 200353615