Add Lox notification for activeLoxId

Currently, if your previous loxId had an invite, and you replace it, the UI in "about:preferences#connection" will show the new loxId as having invites despite the fact that we currently clear invites when TorSettings.bridges.lox_id changes (although that might change in #42453).

This is because the UI reads from the TorSettings.bridges.lox_id before the Lox module has a chance to update its activeLoxId. We should make the UI wait for activeLoxId to update and use that value instead.