Split bpf_helpers.h and bpf_map_def.h from bpf_headers

Moves bpf_helpers.h and bpf_map_def.h out of bpf_headers to restrict
their visibility.
These files are intended for internal use within the tethering module,
but they were exposed to external dependencies.
This CL adds bpf_prog_mainline_headers with strict visibility.

Bug: 424341374
Test: TH
Flag: EXEMPT Android.bp refactoring
Change-Id: Ie35f43ee405e889ea20b367ad16847e49cb5ba61
4 files changed