[lib] Create multimedia unshim spec

Test Plan: Increased min code version in multimedia message spec, logged out and logged in. This causes server to send shimmed message infos to client. The images were displayed correctly because unshimMessageInfos function unshimmed them.

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, atul, subnub

Differential Revision: https://phabricator.ashoat.com/D661