[native] Update local video messages with realized thumbnailURI upon successful upload

Summary:
Modify payload for updateMultimediaMessageMedia action to include realized thumbnailURI upon successful upload when mediaType === "video".

We need to update the thumbnailURI because the temporary local thumbnail gets unlinked after a successful upload.

Depends on D4979

Test Plan: Used the Redux remote dev tools in Chrome to observe the local media message (and the contents of the local media array within) and make sure that the local thumbnailURI got replaced with the "realized" (aka server) thumbnailURI as expected (on dispatch of updateMultimediaMessageMediaActionType)

Reviewers: abosh, jacek, tomek, ashoat

Reviewed By: abosh, tomek

Differential Revision: https://phab.comm.dev/D4980