Fix race between CONNECTIVITY_ACTION and setting default network.

Now that the delay between connectivity changes and CONNECTIVITY_ACTION
has been removed (ag/599650) races between CONNECTIVITY_ACTION and
the setting of the default network become more evident.
In http://crbug.com/441818 Chrome is calling getaddrinfo()
immediately after a device goes from no connectivity to cellular
connectivity, and Chrome is erroneously getting back EAI_NODATA
because netd hasn't yet set the default network for DNS resolutions.

bug:18757162
Change-Id: Ib607dcb3697403272a8c838713a9cb602e9c6820