Count ipv6 tx traffic of clat into 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 here is to 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. atest clatd_test.cpp

Change-Id: I834aef7184a06df89f0db8fee28e2174aa03d817