DO NOT MERGE Always set/remove default routes.

Must clean up default route if a default 3g connection is replaced
by a non-default (ie, mms) connection on teh same interface.

Also stop mucking with all connections dns and routes - do it only
for the connection that has changed.

bug:2865974
Change-Id: Ifdf49080fa0413a4d826813706c809975a562dfa