Correct update notifications

As reported downstream at:
https://bugzilla.redhat.com/show_bug.cgi?id=2009063
https://pagure.io/fedora-workstation/issue/107

There is still some flaw in the notification about new/pending updates.

1. the suspend can delay the notification, depending whether the monotonic time moves on while the machine is suspended or not
2. there had been reset the last update notification timestamp in error after the updates had been received.