Use same intro page system settings icon as in the desktop file

Summary: Use the same system settings icon in the intro page that is used in the desktop file for consistency and compatibilty with other icon themes.

Test Plan: Open the system settings (maybe also with a different icon theme).

Reviewers: Plasma, VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: ngraham, VDG, plasma-devel, Plasma

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D18561