Fix Default route after secondary net use.

Backport of honeycomb-mr1 change 103299 adding a host route for the
default gateway before adding the default route.
Also fixing bug where "already active" condition of APN failed to
propagate the default gateway info, result in a loss of that address.

bug:4175328
Change-Id: I4cf8605b259cf40c3631c7ce6cca4c75c1b22651