Move addUsers/removeUsers from VirtualNetwork to Network

Move uid ranges code from sub class VirtualNetwork to super class
Network. So they can be reused when we support uid ranges on physical
network. No functionality change.

Test: cd system/netd; atest
Test: atest HostsideVpnTests
Change-Id: I6a170264c82418ae1e625c0fc587cd091e56f9dd