Filter debug entries for each vpn

NetworkStats calculation needs to filter out debug entries to
prevent over counting. While NetworkStatsFactory migrates data
usage over a VPN to the TUN network, NetworkStatsFactory does
not filter out debug entries per vpn which will cause debug
entries left and cause exception.

Bug: 152678151
Test: atest com.android.server.net.NetworkStatsFactoryTest
      and verify no exception
Change-Id: I3525edc385b07858b48c7add2d331c4b5a2e84ad