[Wi-Fi] Index Wi-Fi preferences.

This is a WifiTracker2 version of partial part of ag/9980423

- Disable the "Saved network" if the access points is empty.
- Disable the "Wifi data usage" if there is no wifi radio.

Bug: 146823365
Test: manual
      Search 'Saved networks' when there is a saved Wi-Fi network
      and when there is no saved Wi-Fi network.
      Search 'data usage' on the device which supports Wi-Fi and
      on the device which does not supports Wi-Fi.
Change-Id: If46e65585f1b272401666f78fd5e3a4cf49bef6e