Show accurate checked state for system monitor configuration pages

Summary:
Fixed general settings of the widgets "Memory Status", "Hard Disk Space Usage", "Network Monitor", "Hard Disk Monitor".
 - correctly set checked/unchecked state on load.

Reviewers: jriddell, #plasma, ngraham, broulik, davidedmundson

Reviewed By: #plasma, ngraham, davidedmundson

Subscribers: broulik, davidedmundson, ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D17796