Make tethering offload register an ITetheringStatsProvider and
fetch tethering stats from the hardware.
Currently we fetch stats in the following cases:
1. Just after changing upstreams, we fetch stats from the
previous upstream.
2. When we are polled by NetworkStatsService.
(cherry-picked from commit 5a7dea1a8e)
Bug: 29337859
Bug: 32163131
Test: builds, boots
Test: stats appear in tethering logs
Change-Id: If744f2e06cb6a3095a40199936b9afb76eff7b56
Merged-In: If744f2e06cb6a3095a40199936b9afb76eff7b56
4ca0ba8f86