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

res/layout/add_supervised_user.xml

@@ -0,0 +1,32 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+  ~ Copyright (C) 2021 The Android Open Source Project
+  ~
+  ~ Licensed under the Apache License, Version 2.0 (the "License");
+  ~ you may not use this file except in compliance with the License.
+  ~ You may obtain a copy of the License at
+  ~
+  ~      http://www.apache.org/licenses/LICENSE-2.0
+  ~
+  ~ Unless required by applicable law or agreed to in writing, software
+  ~ distributed under the License is distributed on an "AS IS" BASIS,
+  ~ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  ~ See the License for the specific language governing permissions and
+  ~ limitations under the License.
+  -->
+
+<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
+              android:layout_width="match_parent"
+              android:layout_height="match_parent"
+              android:layout_margin="16dp"
+              android:orientation="vertical">
+    <Button
+        android:id="@+id/createSupervisedUser"
+        android:layout_height="wrap_content"
+        android:layout_width="wrap_content"
+        android:text="@*android:string/supervised_user_creation_label" />
+    <TextView
+        android:layout_height="wrap_content"
+        android:layout_width="wrap_content"
+        android:text="@string/placeholder_activity" />
+</LinearLayout>