[lib] Rename setUserAvatar to baseSetUserAvatar in *EditUserAvatarProvider

Summary:
We want devs to to use the platform-specific setUserAvatar wrappers rather than interfacing with setUserAvatar directly... so renaming setUserAvatar to baseSetUserAvatar to make that more clear.

Also left a comment annotating the function, which at least in my editor shows a note on hover/when cursor is focused:

Depends on D8343

Test Plan: flow/eslint/searched codebase for "setUserAvatar".

Reviewers: ashoat, ginsu, rohan

Reviewed By: rohan

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8344