code: public static <T extends AVUser> Observable<T> logIn(String username, String password, final Class<T> clazz) {
      Map<String, Object> params = createUserMap(username, password, null, null, null);
      JSONObject data = new JSONObject(params);
      return PaasClient.getStorageClient().logIn(data, clazz);
    }
docs: logIn in background
  
  @param username
  @param password
  @param clazz
  @param <T>
  @return