Fix Guest string was not translated in UMS.userWithName()

Guest users name was stuck with the language when it was created,
and it wasn't changing with the active language setting.
To solve that, removed name parameter from UM.createGuest() and
created the guest user with a null name, then made changes
in UMS.userWithName() to fill the name of guest user with the active
language's translation when it has a null name.
Creating a guest with a specific name is still supported
through UM.createUser API.

Bug: 185309160
Test: atest com.android.server.pm.UserManagerTest
Change-Id: I2745aed0ea722765a11c6da40fb4159146da54c7
Merged-In: I2745aed0ea722765a11c6da40fb4159146da54c7
(cherry picked from commit c556777c)