simplify tcpTx/RxPackets

On T+ this is mainline code, and the bpf program (netd.c)
does not support collecting tcp specific stats.

(note: this *is* a behaviour change, as it currently
just always returns 0, but UNSUPPORTED/-1 seems to make
more sense)

Test: TreeHugger
Bug: 294604315
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: Ifda15e67d9167a2be98ab6f4e3817ff7c0bb3fea