[Fix] Work around the regression that license.txt is considered unused.

Author: zhanghai

sample/src/main/res/raw/keep.xml

@@ -0,0 +1,22 @@
+<?xml version="1.0" encoding="utf-8"?>
+
+<!--
+ ~ Copyright 2020 Google LLC
+ ~
+ ~ 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
+ ~
+ ~ https://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.
+ -->
+
+<!-- Work around the regression that license.txt is considered unused. -->
+<resources
+ xmlns:tools="http://schemas.android.com/tools"
+ tools:keep="@raw/license" />