PPPoE: fix incorrect condition to set default DNS iface for PPPoE

When interface ppp0 comes up, PPPOEService will set it as default
DNS interface as it think ppp0 is brought up by PPPoE. It is
incorrect since VPN also bring up ppp0.

In the senario VPN up, we don't need to set ppp0 as default DNS
interface as it will use mark policy rule. Setting it to be default
will cause DNS query still use DNS sever which is assigned during
VPN establish procedure after VPN disconnected.

Change-Id: I78595453df831a0cbed35e0fd0b0101535f8bfaf
CRs-fixed: 711085