Added support to visible bg users on UserReference.

Test: atest NeneTest:com.android.bedstead.nene.users.UserReferenceTest

Ignore-AOSP-First: uses new APIs not available on AOSP yet

Bug: 271153404
Bug: 240736142

Change-Id: I7eabc5d8b8fa3f1ff9edf192c3e2900ab8c8c33b