Offline-updates: don't allow to disable

Make packagekit-offline-update a static service.
Users believe by disabling the service, the feature would be disabled,
when in fact systemd still brings the user to the system-update.target
but then doesn't perform any action, most of all not removing the
/system-update link, thus ending in a loop.

Link the new static service into system-update.target.wanted, to ensure
system-update.target triggers offline-update.

https://bugzilla.redhat.com/show_bug.cgi?id=988235