[plasma-desktop] Use new activities icon

Summary:
Use the new icon added in D22155.
{F6948267}

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