Connect metered DHCP hint for Wi-Fi networks.

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