This makes all methods in AlarmNotification safe to call at any time,
independent on whether the KNotification object has already self-destructed
or not. It also avoids creating KNotification objects for still suspended
alarms.

This is an intermediate step to unify the handling of active and suspended
alarms in NotificationHandler, so that we can correctly restore active
alarms after a restart as well, and can finally clean up AlarmNotification
objects properly once dismissed.