Add public.xml to UI module with an empty <public /> node

This is the recommended way to mark all resources as private:
https://developer.android.com/studio/projects/android-library#PrivateResources

PiperOrigin-RevId: 401470108

library/ui/src/main/res/values/public.xml

+<?xml version="1.0" encoding="utf-8"?>
+<!-- Copyright 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.
+-->
+<resources><public /></resources>