Only use mDefaultDns if the network claims to offer Internet

Currently, if a network does not specify DNS servers, we default
it to using 8.8.8.8. This was done because the emulator did not
specify DNS servers. However, it causes queries to fail slowly,
instead of failing fast, on networks that do not have
connectivity to 8.8.8.8.

Bug: 18327075
Change-Id: I0df13ff4a17ee65e640be96695a3af31b020963a