automount: Disable back automount by default

This setting was enabled to make the KCM defaults slightly more consistent, as the partial settings AutomountOnLogin and AutomountOnAttached are enabled by default and the KCM enables the general AutomountEnabled settings from those.

But this seems to be problematic for some use cases (reported on fresh-installed systems like Fedora), so we need to disable it by default.

As the KCM compose the general option from the two partial ones, we need to disable them too, so they're in sync and the Default state is stable.

Follow-up from discussion at !549 (comment 436564)