Facilitate network validation on networks with a per-network PAC.

On networks with a per-network PAC, Network.openConnection(URL) will
fetch using NO_PROXY.  This will fail on networks where Internet
access is only permitted via the proxy.  Always failing network
validation has the potential to disable WiFi auto-join.  Instead
of performing the normal connectivity check, instead attempt to
fetch the PAC, as this is meant to succeed with NO_PROXY.

bug:19143573
Change-Id: Ia482f5c046d338c27daf42571f20851dfa36671c