Update App Engine to runtime=java8 (#2145)

See https://cloud.google.com/appengine/docs/standard/java/migrating-to-java8

java/demo/war/WEB-INF/appengine-web.xml

@@ -2,6 +2,7 @@
 <appengine-web-app xmlns="http://appengine.google.com/ns/1.0">
   <application>libphonenumber-hrd</application>
   <version>1</version>
+  <runtime>java8</runtime>
   <threadsafe>true</threadsafe>
 
   <!-- Configure java.util.logging -->