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/request/Action.java

@@ -26,7 +26,7 @@ import java.lang.annotation.Target;
 public @interface Action {
 
   /** HTTP methods recognized by the request processor. */
-  public enum Method { GET, HEAD, POST }
+  enum Method { GET, HEAD, POST }
 
   /** HTTP path to serve the action from. The path components must be percent-escaped. */
   String path();