[native] Extract transcodeVideo from processVideo

Summary: Pulled transcodeVideo out of processVideo into a separate function in preparation for adding thumbnail step as described in https://phabricator.ashoat.com/D914?id=2722#inline-4817

Test Plan:

1. Force video to transcode by commenting out lib/media/video-utils.js:64-66 and 75:80
2. Send video before applying the changes in this diff, and copy the media mission report
3. Send video after applying this diff, and check to make sure there are no unexpected difference in media mission report

I did this with 3-4 videos and things look to the same as they were before.

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, subnub

Differential Revision: https://phabricator.ashoat.com/D921