code: private boolean startSingleSignOn(Activity activity, String applicationId,
              String[] permissions, int activityCode) {
          boolean didSucceed = true;
          Intent intent = new Intent();
  
          intent.setClassName("com.facebook.katana",
                  "com.facebook.katana.ProxyAuth");
          intent.putExtra("client_id", applicationId);
          if (permissions.length > 0) {
              intent.putExtra("scope", TextUtils.join(",", permissions));
          }
  
          // Verify that the application whose package name is
          // com.facebook.katana.ProxyAuth
          // has the expected FB app signature.
          if (!validateActivityIntent(activity, intent)) {
              return false;
          }
  
          mAuthActivity = activity;
          mAuthPermissions = permissions;
          mAuthActivityCode = activityCode;
          try {
              activity.startActivityForResult(intent, activityCode);
          } catch (ActivityNotFoundException e) {
              didSucceed = false;
          }
  
          return didSucceed;
      }
docs: Internal method to handle single sign-on backend for authorize().
  
  @param activity
  The Android Activity that will parent the ProxyAuth Activity.
  @param applicationId
  The Facebook application identifier.
  @param permissions
  A list of permissions required for this application. If you do
  not require any permissions, pass an empty String array.
  @param activityCode
  Activity code to uniquely identify the result Intent in the
  callback.