Use more appropriate icon that's colorful at small sizes

Summary:
The current icon does not follow the recommended pattern of using preferences- icons
and has a small version that's monochrome, causing the sidebar in System Settings to
inappropriately display some small monochrome icons.

Test Plan:

Reviewers: VDG, ndavis

Reviewed By: VDG, ndavis

Subscribers: kde-utils-devel

Maniphest Tasks: T10165

Differential Revision: https://phabricator.kde.org/D23850