NetdBinderTest.TetherOffloadForwarding - cleanup stats post test

This additionally allows us to check that statistics are correct.

Test: atest netd_integration_test:NetdBinderTest#TetherOffloadForwarding
Bug: 150736748
Original-Change: https://android-review.googlesource.com/1328674
Merged-In: I15a2a9d78ab9d3b5773375a2bd31cb1b2f678c4a
Change-Id: I15a2a9d78ab9d3b5773375a2bd31cb1b2f678c4a