Use the same configs for watching and reading

KColorScheme uses KSharedConfig::openConfig() when no config is specified,
we were watching a KSharedConfig::openConfig("kdeglobals") which seemingly
is not updated instantly because it technically is different even though
effectively it's the same config. Watching the same configs solves the theme
not updating when the global theme is switched
BUG:421745
FIXED-IN:5.21


(cherry picked from commit a197df2c)