Wi-Fi: Ping the driver for wifi-only devices

Wifi-only devices are not capable to find APs with hidden SSID unless a DRIVER COUNTRY command
will send to wifi driver. This change has:

- Send DRIVER COUNTRY command to ping the driver (to correct the seek of hiddens SSIDs)
- Save mCountryCode as uppercase because the wifi_countrycode_values array is in uppercase.
  Otherwise the preference will not be found.

Patchset 7: Rebase
            Do not set mCountryCode if country is null

Change-Id: Ie0f838aef07eaf683b713f7f0e46ff3c751523ab