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

BUG: 449531


(cherry picked from commit 645db7fc)