add bpf prog that accounts for to-be-dropped packets filtered by tc

This functionality is used by go/bandwidth-limiting. Packets that will
be dropped by tc-police do not get accounted for, so they are handed off
to this BPF program to handle interface accounting before being dropped.

Bug: 157552970
Test: builds && bpfloader loads netd.o
Change-Id: I094b4bec2e34b0c059abdd49b5bc54ae2e5b9ce9