code: public BaasResult<BaasUser> loginSync(String registrationId) {
          BaasBox box = BaasBox.getDefault();
          if (password == null) throw new IllegalStateException("password cannot be null");
          NetworkTask<BaasUser> task = new LoginRequest(box, this, registrationId, RequestOptions.DEFAULT, null);
          return box.submitSync(task);
      }
docs: Synchronously logins the user with password and registrationId obtained through gcm.
  
  @param registrationId a registration id for gcm
  @return the result of the request