[native] factor out shared code between Emoji[User/Thread]AvatarCreation into EmojiAvatarCreation

Summary:
The overall approach I took with refactoring this is very similar to how we built the Avatar component. We use EmojiAvatarCreation as the base component that handles all the UI/UX and shared logic, and EmojiUserAvatarCreation handles all the user avatar specific logic and EmojiThreadAvatarCreation handles all the thread avatar specific logic

Depends on D7503

Test Plan: Saving emoji avatars still work as expected, no flow errors

Reviewers: ashoat, atul

Reviewed By: ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7504