[native] Allow LOG_IN ReactNav action to dismiss RegistrationTerms

Summary:
During the registration server call, we want to prevent RegistrationTerms from being dismissed so that eg. a user pressing the Android hardware back button won't make it "go back".

However, we want to allow the LOG_IN ReactNav action to dismiss RegistrationTerms, so that when registration concludes the user is visibly logged in.

This diff solves ENG-8037 by differentiating the LOG_IN action.

Test Plan: Make sure registration flow logs the user in at the end

Reviewers: ginsu

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D11867