Use consistent size icon in OSD

Summary:
The OSD's icon is based on the implicitSize of the IconItem.

For most icons this will be a hardcoded value from
KIconLoader::global()->currentSize

However, since bc33f0470d4d6bcbf30475f85b36705e8a99528b in
plasma-framework we sometimes return that value, sometimes will return the
source size of the underlying image depending on internal paths of how
the icon is loaded and whether it was in the plasma theme or not.

This patch overrides the implicitWidth given to the layout to be the
standard icon size.

BUG: 400083

Test Plan:
Loaded the force kill window OSD.
Icon still looked the same size on breeze.

I didn't reproduce the issue in the bug

Reviewers: KWin, graesslin

Reviewed By: KWin, graesslin

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D16432