When DHCP lease includes vendor info indicating that remote Wi-Fi
network is metered, advise NetworkPolicy. Users can still manually
change the metered flag in Settings.
Also remove any policies belonging to removed Wi-Fi networks, and
teach isNetworkMetered() about ethernet networks.
Bug: 6344821, 6369307, 6365872
Change-Id: I108606c6fddf2d02828fcab011f3a1501415f1bc