Currently, we initialize an HttpAuthPreferences based on local_state() in IOThread, and modify it as the prefs change.  We need to add support for this to the network service.  That's then used to create multiple HttpAuthHandlerFactories, one for each URLRequestContext.

The simplest way to do this in a network_service world it probably to send the immutable prefs and the initial values of the mutable ones to the NetworkService on creation, along with a pipe over which we'll send update for those values that can change (Could just send updates to the top level NetworkService object, but a pipe forces all updates to go through one place), and then have NetworkContexts just grab HttpAuthPreferences from the NetworkService to create their own AuthFactories.

We'll probably want some way to actually authenticate with a server using the new network service APIs before we land anything here, though, so we can run tests.  That gets complicated.