Toggle user switcher in car QS footer.

Toggles display of the user switcher when pressing on the car QS
footer's multi user switch.
Handles expansion and removal of the QS detail so that the view in which the
user switcher displays is only visible when the user switcher is active.

Tested on Mojave
Bug 63593747

Change-Id: I82cb2257131122ab302d64ef47bdc919d222c85d