SystemTray/ExpandedRepresentation: Fix icon and checked state of primary action button

The primary action button is a toolbutton displayed for a contextual action
with the priority set to `PlasmaCore.Action.HighPriority` (this action is always
placed separately from the hamburger menu).

This commit fixes two issues with it:
- the icon of the associated action isn't displayed
- if the associated action is checkable (`checkable` is set to `true`), clicking
  on this button toggles this action twice (checked > unchecked > checked)



(cherry picked from commit 47d355c5)

e04ba8a0 SystemTray/ExpandedRepresentation: Fix icon and checked state of the primary action button