Index wi-fi preferences.

- Add xml file to index "Saved network" and "Wifi data usage"
preferences.
- Disable the "Saved network" if the access points is empty.
- Disable the "Wifi data usage" if there is no wifi radio.

Fixes: 146823365
Test: manual
Change-Id: I049d0463fd58f186dae1d5f54dcc914f52031da7