Option "None" replaced "Open" in Wi-Fi AP Settings of Wi-Fi Hotspot

First option in Wi-Fi AP settings of Wi-Fi Hotspot, "Open", is
changed to "None" since "Open" is not localized.
"None" (@string/wifi_security_none) already in use in other
Wi-Fi related screens and it is already localized in
the existing source code.

Change-Id: Ic79b20e3b0f094e064dd4758d8a03d67a208389a