Clean up some code quality issues

This removes some qualifiers that aren't necessary (e.g. public/abstract on interfaces, private on enum constructors, final on private methods, static on nested interfaces/enums), uses Java 8 lambdas and features where that's an improvement

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=177182945

java/google/registry/util/VoidCallable.java

@@ -25,7 +25,7 @@ public interface VoidCallable {
   void call() throws Exception;
 
   /** Returns the VoidCallable as a {@link Callable} that returns null. */
-  public default Callable<Void> asCallable() {
+  default Callable<Void> asCallable() {
     return () -> {
       call();
       return null;