[applet/systemtray] Use model binding instead of copy
Summary: The effectiveStatus parameter was passed as a copy, use proper binding.
Test Plan: No visible impact
Reviewers: Plasma: Workspaces, Plasma, ngraham, broulik, davidedmundson, apol
Reviewed By: ngraham
Subscribers: apol, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D28185