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/keyring/api/KeyModule.java

@@ -32,7 +32,7 @@ public final class KeyModule {
   /** Dagger qualifier for keys from {@link Keyring}. */
   @Qualifier
   @Documented
-  public static @interface Key {
+  public @interface Key {
     String value();
   }