Wifi: Enable Ad-Hoc (IBSS) network configuration

This adds two public but hidden variables to WifiConfiguration: isIBSS and
frequency. These are used for configuring IBSS mode.

WifiConfigStore is exended to get and set these variables to wpa_supplicant
(via WifiNative).

Change-Id: Iab02c3d738c05c2de036350ea77b78789213e357