NetworkStatsFactory: fix double accounting on IPv6 only networks

For 464xlat scenarios on IPv6 networks, the clatd interface setup
introduces double counting of apps ipv4 traffic. NetworkStatsFactory was
accounting for this on the tx path, but not on the rx path. Also it did
not accounted for the 20 bytes added by the IPv6 header.

This patch subtract correctly the rx and tx traffic from the root uid on
the underlying interface, and also adds correctly the 20 bytes cost per
packet on the stacked interface for 464xlat traffic.

Test: added several new unit tests, based on synthetic data and real
      data also.
Bug: 33681750
Change-Id: I2675643b220acbc6110179fa937d4c313b6f5e32