Add tether offload traffic to interface stats as well.

Currently, we only count add tethering traffic to per-UID
stats, but not to total data usage (i.e., dev and XT stats). This
is correct for software tethering, because all software forwarded
packets are already included in interface counters, but it is
incorrect for hardware offload, because such packets do not
increment interface counters.

To fix this:
1. Add an argument to ITetheringStatsProvider#getTetherStats to
   indicate whether per-UID stats are requested. For clarity,
   define integer constants STATS_PER_IFACE and STATS_PER_UID
   to represent these operations.
2. Make NetdTetheringStatsProvider return stats only if per-UID
   stats are requested. (Otherwise tethering traffic would be
   double-counted).
3. Make OffloadController's stats provider return the same
   stats regardless of whether per-UID stats were requested or
   not.
4. Make NetworkStatsService add non-per-UID tethering stats to
   the dev and XT snapshots. The per-UID snapshots were already
   correctly adding in per-UID stats.

Bug: 29337859
Bug: 32163131
Test: runtest frameworks-net
Test: runtest frameworks-telephony
Change-Id: I7a4d04ab47694d754874136179f8edad71099638