[kio-extras] Use new activities icon

Summary:
Use the new icon added in D22155.
{F6948410}

Test Plan: Open the Dolphin view context menu and visit "activities:".

Reviewers: #plasma, #vdg, ndavis

Reviewed By: #vdg, ndavis

Subscribers: kde-frameworks-devel, #vdg, kfm-devel, #plasma

Tags: #dolphin, #frameworks

Differential Revision: https://phabricator.kde.org/D22172