Revert "Remove the fallback AddSupervisedUserActivity."

This reverts commit 86f3500439.

Reason for revert: Real flow is not ready yet, so we still need the fallback activity.

Change-Id: I34dc5386ff8c18a62b4b11e25fd355a62602329f

src/com/android/settings/users/UserSettings.java

@@ -514,6 +514,13 @@ public class UserSettings extends SettingsPreferenceFragment
                 .setPackage(mConfigSupervisedUserCreationPackage)
                 .addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
 
+        // TODO(b/209659998): [to-be-removed] fallback activity for supervised user creation.
+        if (getActivity().getPackageManager().resolveActivity(intent, 0) == null) {
+            intent
+                .setClass(getContext(), AddSupervisedUserActivity.class)
+                .setPackage(null);
+        }
+
         startActivity(intent);
     }