docs: public static long start(Context context, String tag) {
          final int uid = getUid(context);
          if (uid > 0) {
              long appRxValue = TrafficStats.getUidRxBytes(uid);
              long appTxValue = TrafficStats.getUidTxBytes(uid);
              sReceivedBytes.put(tag, appRxValue);
              sSendBytes.put(tag, appTxValue);
              if (DEBUG) {
                  LogUtils.v(TAG, "start() rxValue=" + appRxValue / 1000 + " txValue=" + appTxValue / 1000 + " uid=" + uid);
              }
              return appRxValue;
          }
          return 0;
      }