[lib] split up useResolvedAvatar

Summary:
Depends on D13718

this diff is responsible for splitting useResolvedAvatar into two new hooks -- one for thread avatars and the other for user avatars. the thread avatar hook will be modified in a later diff to handle setting fc channel avatars as thread avatars

Test Plan: this is just a refactor, confirmed that thread and user avatars still work as expected

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13722