ANDROID: start to re-add xt_IDLETIMER send_nl_msg support

This was reverted in ee6918c6 due to conflicts with
upstream, this first patch is just the minimum necessary to make the netfilter
IDLETIMER target with --send_nl_msg load successfully:

  phone-5.10:/ # iptables-save | egrep IDLETIMER
  -A idletimer_raw_PREROUTING -i rmnet0 -j IDLETIMER --timeout 10 --label 0 --send_nl_msg
  -A idletimer_mangle_POSTROUTING -o rmnet0 -j IDLETIMER --timeout 10 --label 0 --send_nl_msg
  phone-5.10:/ # ip6tables-save | egrep IDLETIMER
  -A idletimer_raw_PREROUTING -i rmnet0 -j IDLETIMER --timeout 10 --label 0 --send_nl_msg
  -A idletimer_mangle_POSTROUTING -o rmnet0 -j IDLETIMER --timeout 10 --label 0 --send_nl_msg

Test: builds, and kernel net tests passes, booted on phone, observed ip{,6}tables loading rules
Bug: 183485987
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I1fe2c4e41a092cc82c3d6d49d1217798b2728bcb
(cherry picked from commit b4355a88)