code: public OmemoElement finish() {
          OmemoHeaderElement_VAxolotl header = new OmemoHeaderElement_VAxolotl(
                  userDevice.getDeviceId(),
                  keys,
                  initializationVector
          );
          return new OmemoElement_VAxolotl(header, ciphertextMessage);
      }
docs: Assemble an OmemoMessageElement from the current state of the builder.
  
  @return OmemoMessageElement