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


(cherry picked from commit 329ddf57)