kcm: use pre-authorization tech instead of interactive auth

the interactive auth was a bit of a stop-gap in absence of being able to forcefully allow rdpserver when enabled. since we have this ability now, let's use it.

this essentially issues a dbus call similar to the cli tool flatpak permission-set kde-authorized remote-desktop org.kde.krdpserver yes

https://develop.kde.org/docs/administration/portal-permissions/