[lib][native] Fix keyserver on master

Summary:
D12696 and D12697 caused issues because they introduced calls to authoritativeKeyserverID() from lib/selectors/user-selectors.js, which is accessed by keyserver.

This diff moves those calls to a separate file that is only accessed by native and web. It also wraps them both in a hook for easy access.

Test Plan: Make sure keyserver runs on master again

Reviewers: tomek, inka, varun, will

Differential Revision: https://phab.comm.dev/D12712