Fix adding of default route.

If you deleted the host routes (started a secondary network like mms, supl
of hipri and then ended it) you would lose the host route to the default
gateway.  Then if you needed to re-add the default gateway route (lost
the connection and removed the default route and then re-established)
you couldn't - can't add a gateway that isn't routable apparently.

This happens if you are in a video chat and lose your connection without
losing the interface (PPP keeps it up for a bit).

Fixed it by having addDefaultRoute first add a hsot route for the gateway
before adding the default route.  This allows the default add to succeed.

bug:3490353
Change-Id: I415e7319832e6456f8757b14c4f79f098a08839b