[native] Introduce NativeEditThreadAvatarProvider

Summary:
Slightly different than what we did with NativeEditUserAvatarProvider. In that case we just needed to pass in platform-specific functions which we could do from root.js.

However, in EditThreadAvatarProvider we use NavContext in order to get the activeThreadID. Introduced NativeEditThreadAvatarProvider to get activeThreadID and pass it to EditThreadAvatarProvider.

In next diff will rename EditThreadAvatarProvider to BaseEditThreadAvatarProvider and will move to lib.

Depends on D8219

Test Plan: Things are still working as expected w/ setting thread avatars.

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8220