[native] Use Alert from old useSaveThreadAvatar in new setThreadAvatar

Summary:
Same diff as D8019, just for thread avatars instead of user avatars.

In the last diff I removed useSaveThreadAvatar, but after comparing the Alert code I think the old code was better.

This diff migrates us to using that Alert code across all callsites.

Depends on D8021

Test Plan: Flow / careful reading, make sure app still runs

Reviewers: ginsu, atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8022