[lib] Refactor useLogIn to be more generic

Summary:
useLogIn is a helper function internal to login-hooks.js. This diff refactors the helper function in preparation for a new use case that will be introduced in the next diff.

Right now useLogIn is coupled with the specific forms of login that it supports; this diff makes it more generic.

Depends on D12112

Test Plan: I tested the flow using staging, with my phone as a primary device and a local simulator as a secondary device

Reviewers: bartek, tomek

Reviewed By: bartek

Differential Revision: https://phab.comm.dev/D12113