update-monitor: stop notifying about unprepared critical updates
GNOME Software is supposed to notify about available updates if:
- The setting to automatically download updates is disabled, or
- The update is prepared
In Software 40, this behavior regressed and Software began improperly notifying about updates before they were prepared. That was mostly fixed in 64585205, except for critical updates. When an update is critical, Software notifies even if automatic downloads are enabled and the update is not yet prepared. This is wrong.
If should_download is true and all_downloaded is false, we should never display a notification, regardless of update criticality.