[lib] Move newly platform-agnostic EditUserAvatarProvider to lib

Summary:
In previous two diffs (D8213, D8215), we made EditUserAvatarProvider "platform-agnostic" so it could be used across web and native.

This diff is just a move + fix imports diff.

Depends on D8215

Test Plan:
Can still select media/upload media to set image user avatar as before.
Also killed keyserver and made sure Alert still appeared as expected on native when upload fails.

Reviewers: ashoat, ginsu

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8216