[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