UserDetailsSettings uses Guest string, not Guest name

The UserDetailsSettings panel, and its title, show the user's UserInfo.name.
This is correct for a regular user (and for the Owner). But for a Guest user,
this is incorrect, and the user_guest string should be used instead.

The difference occurs if the system language is changed after the guest is created;
the word 'guest' should update to the current language, not be frozen to the
language at the time of the guest's creation.

Bug: 185309160
Test: atest UserDetailsSettingsTest
Change-Id: I545aa3e6cc5d00c0bcc49960f37dddd9334b153b