Modify DNS server update methods.

1. Make addDnsServer not add duplicate servers and return a
   boolean value incating whether it changed anything. This is
   consistent with what we do for LinkAddresses and routes.
2. Add a setDnsServers method that sets all the DNS servers to
   the specified collection. This is consistent with what we do
   for LinkAddress.

Bug: 9180552
Change-Id: I5baed09253261b66ea42ae2ea82398118e3ab0ac