keyboard_layout: always expose dbus interface
our plasmoid only listens to signals when the interface was found. also when switching from 2 to 1 layout we'd emit a signal that the layouts changed but then we'd throw away our interface leaving the client wondering what happened to us (and consequently printing warnings because our service wasn't found)
this specifically resulted in the plasmoid not getting layout event changes when switching from 1 to >1
(cherry picked from commit 645db7fc)