UM.isUserNameSet is SystemApi

getUserName is already accessible, so isUserNameSet can be too.
Since it covers a similar topic, it makes sense to also lock it
down with the same permissions (even though the information is
not nearly as sensitive).

Test: atest UserManagerServiceUserInfoTest
Bug: 135582166
Change-Id: I84bc4ec97462b90f0bd1a721e0e41cc14b190e2c