Always displays an option for starting a guest session in the full screen switcher.

If guest is already in the foreground, switcher will just get dimissed upon
the selection of starting a new guest session.

Change-Id: Ia79325a1583f00f35e32e30813163a208acfa0b5
Fixes: 111999338
Fixes: 111453367
Test: Manual in emulator while being logged in as Guest and interacting with the switcher.