[plasma-workspace] Use new activities icon
Summary:
Use the new icon added in D22155.
Test Plan: Open the desktop context menu and also search for "activity" in KRunner.
Reviewers: Plasma, VDG, ndavis
Reviewed By: VDG, ndavis
Subscribers: VDG, plasma-devel, Plasma
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D22167