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

java/google/registry/keyring/api/InMemoryKeyring.java

@@ -155,6 +155,11 @@ public final class InMemoryKeyring implements Keyring {
         "In-memory keyring does not support the retrieval of encrypted data.");
   }
 
+  @Override
+  public byte[] getDecryptedData(String keyName, String encryptedData) {
+    throw new RuntimeException("In-memory keyring does not support decrypting of supplied data.");
+  }
+
   /** Does nothing. */
   @Override
   public void close() {}