Use netlink code to flush rules.

This removes two calls to /sbin/ip on netd startup, which saves
about 70ms. In the future we will be able to use this to flush
routes as well, which will provide similar time savings on every
network destroy operation.

Bug: 34873832
Test: bullhead builds, boots
Test: rules flushed correctly when netd is killed
Change-Id: I4875ac7fec1a92dc5fa2cb68f8fab2a903348c20