[Notifications KCM] Use nice new bell-style notification icon

Summary: We have a nice new icon, added by @trickyricky26 in D18533. Let's us it!

Test Plan: {F6583163, size=full}

Reviewers: #vdg, ndavis

Reviewed By: #vdg, ndavis

Subscribers: trickyricky26, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D18678