We have a few places that open the KCM, with different arguments.
Centralize the implementation in one place.
This makes it easier to switch to invoking systemsettings5 in the future (once https://invent.kde.org/plasma/systemsettings/-/merge_requests/11 is in).
It also makes sure the relevant device is selected when clicking on a pairing notification.
The function is exposed to DBus for the Plasmoid and potential third-party users.
CCBUG: 425660