[native] Add thumbnail generation step to processVideo
Summary: Added thumbnail generation step to processVideo
Test Plan: Got path to thumbnailURI via console.log and made sure the JPG existed/looked correct visually.
Reviewers: ashoat, palys-swm
Reviewed By: ashoat
Subscribers: KatPo, zrebcu411, Adrian, subnub
Differential Revision: https://phabricator.ashoat.com/D914