[lib] Rename EditThreadAvatarProvider to BaseEditThreadAvatarProvider and move to lib

Summary:
This component is now "platform-agnostic" and can be moved to lib. On native it is wrapped by NativeEditThreadAvatarProvider which passes in platform-specific props.

Depends on D8220

Test Plan:
Renamed and moved file and let IDE auto-fix imports.

Everything continues to work as expected.

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8221