Only export the org.kde.keyboard DBus service if there are at least two layouts

Summary:
It doesn't make much sense to export the DBus service if there is nothing
one can do with it.

Test Plan: Added test case

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D4562