Use consistent default icon in UserManager and Kicker

Summary:
Both Kicker and UserManager when there is no icon set fallback to the
icon "user-identity". However because we basically maintain two distinct
icon themes for Plasma and not Plasma they ended up looking completely
different as in the bug report.

BUG: 370362

Reviewers: #plasma, hein

Reviewed By: #plasma, hein

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4249