[native] Avoid transition delay during upload

Summary:
Fixes ENG-3615. I discovered that the problem was in our departing vs current source logic in multimedia.react.js. Both sources caused onLoad to be triggered. Because departing source is usually local URI, it was loaded much faster, triggered onLoad which removed itself way before the current source was actually loaded.

Added boolean to indicate that onLoad should not be triggered for departing source.

Depends on D7765

Test Plan: Artifically disabled cache and delayed decryption in encrypted-image.react.js and sent an encrypted photo. The local image was displayed instead of a spinner until decryption finishes. After leaving and re-entering the thread, the spinner was displayed until the image was decrypted.

Reviewers: ashoat, atul

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7782