KColorSchemeManager: don't override color scheme set by platform theme

This allows to have Qt platform theme setting the color scheme before
it gets set and overriden by KColorSchemeManager. This is useful for 3rd
party platform themes like QGnomePlatform, allowing them to set a color
scheme that would complement QPalette. Since many KDE apps rely on using
KColorScheme, it can happen that they will use a color from KColorScheme
that doesn't correspond to QPalette set through the platform theme and
end up being broken.

BUG: 447029