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