Ignore ipv6 tx traffic on clat uid.

Currently, 464xlat counts its ipv6 tx traffic into root uid.
When user is making ipv4 upload over ipv6-only network, ipv4
tx traffic may sometimes be counted faster then ipv6 tx
traffic.

Thus, NetworkStatsService may detect non-monotonic values due
to 464xlat adjustment.

So the solution is that: for clatd, make ipv6 tx traffic counts
into clat uid, and then ignore it in the framework side.

Bug: 118602783
Test: 1. manually verify clatd traffic on clat uid.
      2. runtest frameworks-net

Change-Id: Ifb478b79e3e281918c70e16d1f90682c78f33db1