[native] introduce useSaveUserAvatar hook

Summary:
To better prepare the EmojiAvatarCreation component to handle both user and thread avatars I want to factor out this User avatar specfic function into a hook in the avatar util file so that the component won't be as cluttered when we extend to thread avatars

Depends on D7423

Test Plan: flow and I am still able to save my avatar with the expected behavior

Reviewers: ashoat, atul

Reviewed By: ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7424