[System Tray] Always draw the highlight behind the icon
There was a slight issue introduced with D28109: the systray highlight can now cover up the icons it's highlighting.
This isn't really an issue with Breeze because it uses a line for a highlight, but it is for themes that use a filled-style highlight.
I set a z value for the highlight that should ensure it's now drawn only behind the icon.
Tested with the Mondrian Plasma theme
Reviewers: kmaterka, Plasma
Reviewed By: kmaterka
Differential Revision: https://phabricator.kde.org/D29380