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

Original change: https://android-review.googlesource.com/c/platform/frameworks/base/+/1413610

Change-Id: I70b49d0a10b80f378dcd2ba78b5bac848e40e5c9
Merged-In: Iabe265467044fea9fa437674ca29ffc7bcdefe3b
(cherry picked from commit 9cd671a4)