[User Switcher] remove add guest user...

for secondary user. This only applies to the newly refactored User
Switcher.

Fixes: 254913348
Test: Added unit tests for user interactor.
Test: Add secondary user and guest as system user. Switch to secondary user. Look at bouncer user
switcher and fullscreen user switcher and observe no guest option.
Test: Set add user from lockscreen to on. Switch to secondary user.
Look at bouncer user switcher and fullscreen user switcher and observe
that there is an add guest option along with add user option.

Change-Id: I4cf3121c9a8aefbb78d62f0347511470940abff4