[native] Rename RegistrationButton state prop to variant

Summary:
In a later diff I'll introduce a new variant for this, and with the new variant I figured the term "variant" made more sense than "state" (it's no longer specific to loading state).

Depends on D7724

Test Plan: Flow

Reviewers: atul, ginsu

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7725