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
