Fetch tethering offload stats.

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.

Bug: 29337859
Bug: 32163131
Test: builds, boots
Test: stats appear in tethering logs
Change-Id: If744f2e06cb6a3095a40199936b9afb76eff7b56