[kcms/keys] Make saving sync
Summary:
If it's async it will not save when changing modules, or quitting kcmshell, see T13140.
BUG: 421333
Test Plan: Make a change, change module in systemsettings. Changes are saved.
Reviewers: broulik, Plasma, davidedmundson
Reviewed By: Plasma, davidedmundson
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D29635