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/tools/CreatePremiumListCommand.java

@@ -36,7 +36,7 @@ public class CreatePremiumListCommand extends CreateOrUpdatePremiumListCommand {
   }
 
   @Override
-  ImmutableMap<String, ? extends Object> getParameterMap() {
+  ImmutableMap<String, ?> getParameterMap() {
     if (override) {
       return ImmutableMap.of("override", override);
     } else {