Have authhelper define config file paths on its own
Suggested for D26813. Prerequiste if user-manager is to not send any SDDM config file path on its own.
The config file paths are now defined in the authhelper as opposed to being sent in a variant map from the main kcm file.
I also edited paths in that main file because they were wrong.
This does not yet solve bug 414396.
Test Plan: Testing with D26813 I managed to write to the config file without user-manager defining the path.
Reviewers: Plasma, davidedmundson
Reviewed By: Plasma, davidedmundson
Differential Revision: https://phabricator.kde.org/D27142