[lib] Rename setThreadAvatar to baseSetThreadAvatar in EditThreadAvatarProvider

Summary:
Thread avatar equivalent of D8344

We want devs to use the platform-specific setThreadAvatar wrappers rather than interfacing w/ setThreadAvatar from provider directly... so renaming to make that more clear.

Depends on D8758

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

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8759