Added UserManagerInternal.getDisplayAssignedToUser(userId).

Currently, to check if a user is visible, internal services
need to get a context to the user, which is not only more work
(and more expensive), but would not work if the user doesn't exist.

Test: called by Car stack
Bug: 243864134

Change-Id: I25d0915c94ac670cfd126b94ec988d2c4107bc1c