Track tethering active sessions counts

This commit implements the necessary counters to bookkeep
max connection count, which will be used in follow-up changes
to monitor and upload active tethering session metrics.

The feature is disabled by default and will be gradually
rolled out.

Test: atest TetheringTests:com.android.networkstack.tethering.BpfCoordinatorTest
Bug: 354619988
Change-Id: Iaf251b0ec08bde95c1b12df022db2e03179309de
5 files changed