[plasma-desktop] Use new activities icon
Summary:
Use the new icon added in D22155.
Test Plan: Open the context menu of an activity pager and also add an activity manager widget.
Reviewers: Plasma, VDG, ndavis
Reviewed By: VDG, ndavis
Subscribers: VDG, plasma-devel, Plasma
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D22168