Fix network usage stats on 464xlat tethered.

Usage stats corrections for 464xlat in NetworkStatsFactory are not applied
to tethered traffic. Add adjustments in NetworkStatsService. After
migrating external callers off NetworkStatsFactory, we will be able to
only apply adjustments in NetworkStatsService and remove stacked
interface tracking from NetworkStatsFactory.
Bug: 72107146
Fixes: 72107146
Test: runtest frameworks-net & manual - checked corrected network usage

Change-Id: I5ce450e616b4fddf21f2a491fe5d0c9e9f969bda