Revert "Drop dbus activation stub service"

This reverts commit 07125d24.

In contrast to what is claimed in #13396 dbus-broker apparently does
care for the service file to be around, and otherwise will claim
"Service Not Activatable" in the time between systemd starting up the
broker and connecting to it, which the stub service file is supposed to
make go away.

Reverting this makes the integration test suite pass again on host with
dbus-broker (i.e. current Fedora desktop).

Tested with dbus-broker-21-6.fc31.x86_64.

(cherry picked from commit b895fa08)
(cherry picked from commit faba5b2b)