Set OrganizationDomain before registering the DBus service

The OrganizationDomain is used by KDBusAddons to generate the service
name used to register on the session bus so we need to set it up before
we try registering on the bus.

See: https://bugs.kde.org/show_bug.cgi?id=470246
See: https://github.com/flathub/org.kde.kleopatra/issues/34

CCBUG: 470246
(cherry picked from commit 66753b1e0d306d589f17c5dfd55e51e45d818ec2)