Enable Google Java Format in the Spotless plugin.

Change-Id: Ib2cbe5bb6a6eaaa11bc34b13653776a9ea3ea921
diff --git a/build.gradle b/build.gradle
index 518e861..f436299 100644
--- a/build.gradle
+++ b/build.gradle
@@ -55,6 +55,11 @@
   scala {
     scalafmt()
   }
+
+  java {
+    googleJavaFormat()
+    removeUnusedImports()
+  }
 }
 
 sourceCompatibility = 13