Fixes user switcher name and icon regressions.

In the full-screen user switcher:
- The guest action in the Add menu is now "Add Guest" instead of just
  "Guest"
- The add user icon is updated to what it's meant to be for large
  screens
- In guest mode, the guest user label is back to reading "exit guest"
  instead of just "guest"

Fix: 253623761
Test: manually verified on a large screen device that everything
mentioned above is correct.

Change-Id: I06cd4d21d3303765aa70b81bd714933f8da433f2