code: @Override public void onActivityForResult(int requestCode) {
          if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
              if (requestCode == OVERLAY_PERMISSION_REQ_CODE) {
                  if (isSystemAlertGranted()) {
                      permissionCallback.onPermissionGranted(new String[]{Manifest.permission.SYSTEM_ALERT_WINDOW});
                  } else {
                      permissionCallback.onPermissionDeclined(new String[]{Manifest.permission.SYSTEM_ALERT_WINDOW});
                  }
              }
          } else {
              permissionCallback.onPermissionPreGranted(Manifest.permission.SYSTEM_ALERT_WINDOW);
          }
      }
docs: used only for {@link Manifest.permission#SYSTEM_ALERT_WINDOW}