Use RequiresFlagsEnabled for flag dependent tests.

Also need to instantiate CheckFlagsRule for RequiresFlagsEnabled to
work.

Bug: 393987170
Flag: com.android.documentsui.flags.support_visible_background_user
Test: DocumentsUIUnitTests:UserManagerStateTest
Change-Id: Ib4785cf746c902d409fc35724150fcce200b96e0