[native] Introduce generateThumbnail, rename FFmpeg:process to FFmpeg:trancodeVideo
Summary: Introduce generateThumbnail and rename FFmpeg:process to FFmpeg:transcodeVideo
Test Plan: verified thumbnail was generated by checking the console.log'd output path.
Reviewers: ashoat, palys-swm
Reviewed By: ashoat
Subscribers: KatPo, zrebcu411, Adrian, subnub
Differential Revision: https://phabricator.ashoat.com/D899