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

javatests/google/registry/testing/Providers.java

@@ -24,10 +24,6 @@ public final class Providers {
    *
    */
   public static <T> Provider<T> of(final T instance) {
-    return new Provider<T>() {
-      @Override
-      public T get() {
-        return instance;
-      }};
+    return () -> instance;
   }
 }