Cache Wi-Fi state in worker thread

- The WifiManager.getWifiState() is a blocking API, It may not be
  suitable for use on the UI thread.

- To avoid blocking the UI thread, we should cache the Wi-Fi state
  from the WIFI_STATE_CHANGED broadcast to return immediately.

Bug: 226039078
Test: manual test
make RunSettingsLibRoboTests ROBOTEST_FILTER=WifiStateWorkerTest

Change-Id: Ia0443824975f01099e52f777c15468f30285f4c1