code: public void checkAuthStatus(SimpleLoginAuthenticatedHandler handler) {
      if(androidContext != null) {
        SharedPreferences sharedPreferences = androidContext.getSharedPreferences(Constants.FIREBASE_ANDROID_SHARED_PREFERENCE, Context.MODE_PRIVATE);
        String jsonTokenData = sharedPreferences.getString("jsonTokenData", null);
        if(jsonTokenData != null) {
          try {
            JSONObject jsonObject = new JSONObject(jsonTokenData);
            attemptAuthWithData(jsonObject, handler);
          }
          catch (JSONException e) {
            handler.authenticated(null, null);
          }
        }
        else {
          handler.authenticated(null, null);
        }
      }
      else {
        handler.authenticated(null, null);
      }
    }
docs: Check the authentication status. If there is a previously signed in user, it will reauthenticate that user.
  
  @param handler Handler for asynchronous events.