Make PrivateAddressCoordinator ignore vpn network

If vpn app make its netmask as 0, PrivateAddressCoordinator
would not able to find suitable address for tethering downstream.
Since tethering do not support vpn as upstream, just ignore vpn
in PrivateAddressCoordinator.

Bug: 166365863
Test: atest TetheringTests
      atest CtsTetheringTest
Change-Id: Iabe265467044fea9fa437674ca29ffc7bcdefe3b