Wifi: Add support for enabling Wifi while in airplane mode.

If the new system settings value for AIRPLANE_MODE_TOGGLEABLE_RADIOS
contains RADIO_WIFI, then the user will be allowed to enable Wifi
while in airplane mode.
Turning on airplane mode will still disable Wifi, but the user will
be free to reenable it in the Settings app.

Signed-off-by: Mike Lockwood <lockwood@android.com>