ANDROID: core of xt_IDLETIMER send_nl_msg support

This was reverted in ee6918c6 due to
conflicts with upstream, and this attempts to reapply the majority
of that change in an upstream compatible fashion.

Test: builds, and kernel net tests passes, booted on phone,
  but no real testing
Bug: 183485987
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I0fa1da3cb38d37c6888444dc36d49e6d1828a855