Change icon for Activities config

Summary:
KCMs should have color icons while widgets should have monochrome icons, so I have changed the name of the KCM icon.

Depends on D18529

Reviewers: #vdg, #plasma, ngraham

Reviewed By: #vdg, ngraham

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D18531