Save the correct value of noborder property

Summary:
We can't use Client::noBorder while saving the desktop session because
the returned value is not necessarily set by the user.

BUG: 403948

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D18756