[lib] Rename updateImageUserAvatar to baseUpdateImageUserAvatar in EditUserAvatarProvider

Summary:
Effectively the same as D8344, but for updateImageUserAvatar.

We want devs to use the platform-specific updateImageUserAvatar wrappers rather than interfacing with updateImageUserAvatar directly... so renaming with base prefix to make that more clear.

Also left a comment.

Depends on D8348

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

Reviewers: ashoat, ginsu, rohan

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8360