Only trigger keyboard layout OSD if KWin manages input

Summary:
On platforms where KWin does not manage input, that is does not
use libinput, KWin gets the keyboard layout from another place, e.g.
the X server. In that case KWin should of course not trigger the
layout changed OSD if KWin thinks (for whatever reason) that the
layout changed.

BUG: 367637

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D2521