Expose the functionality to decrypt given data using keyring

This allows us to provide the keyring a blob of encrypted data and a key name, and have it decrypt it for us.

Also fixed javadoc length in Keyring.java. It seems like it was using a 80-character length limit.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=222995542

javatests/google/registry/testing/FakeKeyringModule.java

@@ -156,6 +156,11 @@ public final class FakeKeyringModule {
             "Fake keyring does not support the retrieval of encrypted data.");
       }
 
+      @Override
+      public byte[] getDecryptedData(String keyName, String encryptedData) {
+        throw new RuntimeException("Fake keyring does not support decrypting of supplied data.");
+      }
+
       @Override
       public void close() {}
     };