Move netlink updates to a new NetlinkTracker class.

This will allow us to use this logic on other link types such as
Ethernet, and will also avoid increasing the complexity of
WifiStateMachine as we get more information such as routes and
DNS servers from netlink.

Bug: 9180552
Change-Id: I79034827cb0c5f24d783d7243e04b69e12aa3cc6