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/flows/ServerTridProvider.java

@@ -24,5 +24,5 @@ import google.registry.model.eppcommon.Trid;
 public interface ServerTridProvider {
 
   /** Creates a new server Trid. */
-  public String createServerTrid();
+  String createServerTrid();
 }