[User Switcher] Change layout of menu.

Change the layout of the menu in the fullscreen user switcher such that
manage users is in its own partiion. Also add some dividers between the
items.

Fixes: 253609111
Test: I tested by going to the fullscreen user switcher menu on a large
screen device. I have changed the orientation of the activity from
portrait to landscape with the menu open. I have open and closed the
menu repeatedly. I have clicked on all of the options of the menu as
well.

Change-Id: Ib6e74197f4bd6a28f1ae6371189c805ec5fcbf6f