Say a phone is disconnected if we can't find it.

isDataDisconnected(subId) was returning false if it couldn't
find a phone servicing the given subscription.  That seems
wrong and led to ServiceStateTracker thinking it was connected
if there was no SIM inserted.  That in turn led to very slow
AirplaneMode engagement.

bug: 21427496
Change-Id: I762e5dae6bc4263e846245ddd49097e8bc204e12