Improve the Notfication Bell Icon by using the KAlarm design

Summary:
BUG: 400570

FIXED-IN: 5.55

Improve the preferences-desktop-notification-bell icon by using the design from the KAlarm app icon.

Test Plan:

Reviewers: VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: abetts, ngraham, ndavis, djarvie, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D18533