[Look and Feel KCM] Write schemeName last

We copy the .colors file's contents into kdeglobals and I have seen many of them containing bogus

[General]
ColorScheme=Some Other Scheme
which makes no sense. Ensure the ColorScheme that ends up in kdeglobals is the scheme defined by the look and feel package.

BUG: 403774
FIXED-IN: 5.15.0

Differential Revision: https://phabricator.kde.org/D18754