[WifiDialog] Allow adding unspecified AP

In WifiDialogActivity, handle the case where the given
accessPointState is null, and show the Wi-Fi dialog allowing users
to enter the SSID and other credentials. This will be used by setup
wizard when user selected "Add new network" item.

Bug: 23392285
Change-Id: Ica2650d9e829d73281bc9c504c3005d9c5750d31