Bug 1000323 - Reauthenticate the user when disabling Find My Device.

We only get a wake up request when enabled turns to true, not false, because the FMD app has nothing to do when disabled, so we don't need it to run (though that may change as part of bug 1030249). But if the FMD app is already running and FMD is disabled, then we want to know about that, and that's what this observer does.