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

-------------
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();
   }
 

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) {

java/google/registry/keyring/kms/KmsKeyring.java

@@ -41,7 +41,7 @@ import org.bouncycastle.openpgp.PGPPublicKey;
  */
 public class KmsKeyring implements Keyring {
 
-  static enum PrivateKeyLabel {
+  enum PrivateKeyLabel {
     BRDA_SIGNING_PRIVATE,
     RDE_SIGNING_PRIVATE,
     RDE_STAGING_PRIVATE;
@@ -51,7 +51,7 @@ public class KmsKeyring implements Keyring {
     }
   }
 
-  static enum PublicKeyLabel {
+  enum PublicKeyLabel {
     BRDA_RECEIVER_PUBLIC,
     BRDA_SIGNING_PUBLIC,
     RDE_RECEIVER_PUBLIC,
@@ -63,7 +63,7 @@ public class KmsKeyring implements Keyring {
     }
   }
 
-  static enum StringKeyLabel {
+  enum StringKeyLabel {
     BRAINTREE_PRIVATE_KEY_STRING,
     ICANN_REPORTING_PASSWORD_STRING,
     JSON_CREDENTIAL_STRING,