[applets/systemtray] Fix value read in some onXyxChanged connections

Summary: Some `onXyzChanged` signals do not send value - fix connection to use value directly from the sender.

Test Plan: Disable or enable plasmoid in System Tray setting - it should work correctly.

Reviewers: #plasma_workspaces, #plasma, ngraham, apol, broulik, davidedmundson

Reviewed By: apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D29356