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
Wi-Fi: Pass default country code to driver

This patch reverts partially the change http://review.cyanogenmod.org/#/c/28918/8, and
passing the default country code ("GB") to the driver instead of null.

Change-Id: I7d7115155a0a52261931ffaddaf35f316d095800
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>
SystemUI: Remove the dark pixel artifacts from the back arrow drawable

The arrow had some mostly transparent pixels that made it look
kind of like the device had defective pixels. Found by Cyril Mottier.

Change-Id: Ia78453868ebe7e3291d01c96b9beafc78ea3cdcc