Create build.gradle (GoogleCloudPlatform#2543)

Try and convince RenovateBot to update gradle files.
- [ ] Tests pass
- [ ] Appropriate changes to README are included in PR
- [ ] API's need to be enabled to test (tell us)
- [ ] Environment Variables need to be set (ask us to set them)

Author: lesv

build.gradle

@@ -0,0 +1,25 @@
+// Copyright 2020 Google Inc.
+//
+// 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.
+//
+////////////////////////////////////////////////////////////////////////////////
+
+
+// Fake Gradle.build file to convince RenovateBot this repo uses gradle.
+subprojects {
+  apply plugin: 'java'
+
+  repositories {
+    mavenCentral()
+  }
+}