[native] naively split EmojiAvatarCreation into EmojiUserAvatarCreation and EmojiThreadAvatarCreation

Summary:
Copied the code in EmojiAvatarCreation and pasted it into both EmojiUserAvatarCreation and EmojiThreadAvatarCreation. Also updated the navigators and imports to use these two new components.

There is a bunch of shared code in EmojiUserAvatarCreation and EmojiThreadAvatarCreation, but the next diff will handle factoring out all the shared code back into EmojiAvatarCreation

Test Plan: Saving emoji avatar still works, no flow errors

Reviewers: ashoat, atul

Reviewed By: ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7503