Do not start RILReceiver if wifi-only.

By not starting RILReceiver we don't waste any cycles
trying to communicate with a vendor ril which may
not exist. This allows a wifi-only device to operate
as expected and the system to gracefully handle the
missing radio as ril requests will report RADIO_NOT_AVAILABLE.

Change-Id: I6c6f60830486c5f0447b3b9eb44c8b1f2d70c517