Refactoring the User Switcher layout files to use dimensions in

dimens_car.xml and colors_car.xml so that overlays will be easier.
Also, added a dialog when adding a user.  Fixed the issue where coming
back to the user picker makes the buttons unresponsive

Test: tested on Mojave
Bug: 78244241, 78309816, 78297334,
Change-Id: Iec44eb93ec4456be6d39d08940bb6ba2050eac7a