kcm/rules: Use the new KCM namespace for standalone launch
After commit c1d0aa18, the namespace where the KCM is installed changed to `plasma/kcms/systemsettings/`. We need to point to this new plugin location when the KCM is launched via Alt-F3 > "Window Specific Properties" Also let's set the full plugin location from CMake instead of harcoded in C++ so it is easier to keep it in sync. BUG: 454066 FIXED-IN: 5.25.0