Remove support for Root identity singletons and rely on the all_users bit to be set. + test

mojo/shell/tests/connect/connect_test_app_manifest.json

Index: mojo/shell/tests/connect/connect_test_app_manifest.json
diff --git a/mojo/shell/tests/connect/connect_test_app_manifest.json b/mojo/shell/tests/connect/connect_test_app_manifest.json
index 7c4592271915746eab0170e12bc0284bf8447c9a..93b97820e3ec35d7580785ae697bd7eabd6268bc 100644
--- a/mojo/shell/tests/connect/connect_test_app_manifest.json
+++ b/mojo/shell/tests/connect/connect_test_app_manifest.json
@@ -9,7 +9,7 @@
         "classes": [ "class" ],
         "interfaces": ["mojo::shell::test::mojom::ConnectTestService"]
       },
-      "mojo:shell": { "classes": [ "user_id" ] }
+      "mojo:shell": { "classes": [ "user_id", "all_users" ] }
     }
   }
 }