[NETD-BPF#3] Relocate Tethering/bpf_progs and rename bpf_tethering_headers

The folder is currently used by tether offload only. Because we will
move netd.c and clatd.c to it, the folder should be moved to the upper
tier.

Also, rename bpf_tethering_headers to bpf_connectivity_headers so that
other connectivity code besides to tethering are justified to use it.

Bug: 202086915
Test: atest FrameworksNetTests
Change-Id: I95943c6e909f1fdca12604ef0c55d67c39ca686b
7 files changed