Set default name when creating a supervised user in fallback activity.

Fallback activity for supervised user creation was creating a supervised user
without a name (null), now creates it with a default name.
Also put an explanation on the activity stating it's just a placeholder.

Bug: 215232217
Bug: 207495450
Test: croot && make RunSettingsRoboTests -j40 ROBOTEST_FILTER="com.android.settings.users.UserSettingsTest"

Change-Id: I8090fde38155817ac2b4f88271abe8a0ad1ce747

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

@@ -65,7 +65,9 @@ public class AddSupervisedUserActivity extends Activity {
     }
 
     private void createUser() {
-        final NewUserRequest request = new NewUserRequest.Builder().build();
+        final NewUserRequest request = new NewUserRequest.Builder()
+                .setName(getString(R.string.user_new_user_name))
+                .build();
 
         final AlertDialog pleaseWaitDialog = new AlertDialog.Builder(this)
                 .setMessage(getString(R.string.creating_new_user_dialog_message))