[kactivities] Use new activities icon
Summary:
Use the new icon added in D22155.
Test Plan: Open the activities overview.
Reviewers: Plasma, VDG, ndavis
Reviewed By: VDG, ndavis
Subscribers: VDG, kde-frameworks-devel, Plasma
Tags: Frameworks
Differential Revision: https://phabricator.kde.org/D22173