Use system captive portal URL

The captive_portal_server setting is deprecated, and hard-wiring it as a
string is also probably a bad idea. Since it's deprecated, this code
will miss an override via the modern settings. This also hard-wired a
slightly different fallback than ConnectivityService, and simply
adjusting it here wouldn't stop that from happening again. It's best to
just use the privileged API for obtaining the proper value. The choice
of LOCAL_MAC_ADDRESS as the permission for this is a bit strange, but
that's what is required, probably because a better fit didn't exist.

Change-Id: I78d07d5ff96598cebf6d4ed032fe1516791836fe