Speed up network transition when WiFi disconnects

Before falling back to cellular we used to first delete all the
network routing tables and rules for WiFi.  This isn't necessary
and can take significant time as it requires a lot of netd
shelling out to ip and ip[6]tables to flush routes and remove
the incoming packet mark rule.  Instead have netd delete all the
network routing tables and rules after we've either fallen back
to cellular or at least kicked off a cellular connection attempt.

Bug: 21932815
Change-Id: Iabac4a8b962492682df3073cc41a12e35bc9f1bb