Changed list metered-networks so it returns all networks.

Prior to this change it was relying on NMPS.getNetworkPolicies(), which
would not return networks that never had any policy attribute (like
metered) set.

This change fix this problem by also querying the WifiManager for the
list of saved wi-fi networks. In fact, the new logic is very similar to
what's done on SettingLib's NetworkPolicyEditor.

BUG: 26685616
Change-Id: I42f37de3a7367718036f45b15c6b5d37b67e7d01