[native] Replace generic useSaveThreadAvatar with new generic setThreadAvatar

Summary:
Unifying codepaths here, similar to what D8018 did for user avatars.

In a later diff I'll need to make sure that activeThreadSelector works with the EmojiThreadAvatarCreation screen.

Depends on D8020

Test Plan: Test setting emoji avatar for thread from iOS simulator

Reviewers: ginsu, atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8021