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: Iec7617f5712c1b15a480b155d7eb5503208fd3df