Consolidate [Web/Native/Base]EditUserAvatarProvider into BaseEditUserAvatarProvider

Summary:
As of this diff all of the platform-specific functionality has been moved out of BaseEditUserAvatarProvider, which allows us to consolidate the three previous providers into one.

This concludes most of the refactoring work aimed at decoupling avatar-hooks and *AvatarProviders.

Next diff will involve renaming BaseEditUserAvatarProvider to EditUserAvatarProvider (was initially supposed to be part of this diff, but all the import changes and whatnot made it messy)

Depends on D8346

Test Plan: Still able to update avatars as expected on both native and web.

Reviewers: ashoat, ginsu, rohan

Reviewed By: rohan

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8347