[applets/systemtray] Rework of "expanded" state handling

Refactor and simply logic of "expanded" state when dialog with active applet or list of hidden applets is shown.

Fixes a problem with keyboard shortcut activation.
It also fixes a bug when dialog closes on "go-back" button click.

BUG: 427685
FIXED-IN: 5.21
CCBUG: 400278