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
Wifi: Add IBSS supported method

Add method isIbssSupported() to WifiManager and related classes to query wether
the driver supports IBSS mode. The query is answered by wpa_supplicants new
command "get_capability modes".

It will only return a correct result after WIFI_STATE_ENABLED has been
reached.

Change-Id: I93aa2fdff94cd897c91ec3beb7e58940d1de7b72
Wifi: Get the list of supported channels

Get the list of supported channels/frequencies from wpa_supplicant and provide
them thru WifiManager.getSupportedChannels();

One channel is represented thru the class WifiChannel which includes channel
number, frequency in MHz and a flag if IBSS mode is allowed on this channel.

Like isIbssSupported() we can only expect a result after WIFI_STATE_ENABLED has
been reached.

Change-Id: I9fc34642b9a0047f6871b180a3a7a69d47b7e2ff