Always try to set/remove default routes

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

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

bug:2865974
Change-Id: I589a0b2768b5e67b608fde181e7ddbd7fce4f491