docs: static String setRegistrationId(Context context, String regId) {
          final SharedPreferences prefs = getGCMPreferences(context);
          String oldRegistrationId = prefs.getString(PROPERTY_REG_ID, "");
          int appVersion = getAppVersion(context);
          Log.v(TAG, "Saving regId on app version " + appVersion);
          Editor editor = prefs.edit();
          editor.putString(PROPERTY_REG_ID, regId);
          editor.putInt(PROPERTY_APP_VERSION, appVersion);
          editor.commit();
          return oldRegistrationId;
      }