Keep address rotation alarm when all clients are gone

When all clients unregister, we still need to keep the alarm. It's not
re-created when new clients are registered, so i.e. restarting a scan
would cause a crash.

Test: start scan multiple times
Bug: 145832107
Change-Id: Ibe9f85f789b10bbb4ade463501fe6551f269f4f8