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/PgpHelper.java

@@ -45,7 +45,7 @@ public final class PgpHelper {
    * Narrowed key search requirements.
    * @see PgpHelper#lookupPublicKey
    */
-  public static enum KeyRequirement { ENCRYPT, SIGN, ENCRYPT_SIGN }
+  public enum KeyRequirement { ENCRYPT, SIGN, ENCRYPT_SIGN }
 
   /** Converts {@code publicKey} to bytes. */
   public static byte[] convertPublicKeyToBytes(PGPPublicKey publicKey) {