[lib][native] Introduce isLoggedInToAuthoritativeKeyserver

Summary: This is a simple utility. I was about to introduce a new call to isLoggedInToKeyserver(authoritativeKeyserverID()), so I decided it would make sense to factor it out.

Test Plan:
This should be a simple no-op refactor, so my testing was limited:

1. Flow
2. Careful inspection of the code
3. The changes to NavigationHandler were tested as part of the stack

Reviewers: tomek, inka

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D12696