[native] Introduce NativeEditUserAvatarProvider

Summary:
Wraps BaseEditUserAvatarProvider and passes in native-specific props.

Sorry this diff is a little noisy, the main changes are in root.react.js and native-edit-thread-user-avatar-provider.react.js, rest are mostly import fixes.

Depends on D8216

Test Plan: Everything continues to work as expected on native.

Reviewers: ashoat, ginsu

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8217