Make BPF tethering offload coordinator, BpfCoordinator,
registers a network stats provider, BpfTetherStatsProvider, and
provide the tethering stats from the BPF map.

Bug: 150736748
Test: new test BpfCoordinatorTest

Original-Change: https://android-review.googlesource.com/1256189
Merged-In: I22e71f87b67668f7e733e4f215d93bf5b2c9380d
Change-Id: I22e71f87b67668f7e733e4f215d93bf5b2c9380d