[lib] Expose isAuthorized property in Tunnelbroker context

Summary:
Small change to expose this property - it's useful for secondary device to see if credentials switch (unauth -> auth) has been finished

Depends on D11291

Test Plan: Flow. Logic tested in one of further diffs

Reviewers: kamil, michal

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11292