Track IP addresses in WifiStateMachine.

This makes WifiStateMachine listen for interface address
changes. When an address changes, WifiStateMachine will update
its LinkProperties to match reality and notify the system that
the link configuration has changed. This allows wifi to keep
track of IPv6 addresses as well (previously it only knew about
the IPv4 address passed in by DHCP).

IP addresses are obtained directly from the kernel and always
match those configured on the interface, so we treat them as
authoritative and ignore addresses passed in via DHCP (by the
time DHCP passes them in we've already been notified by the
kernel anyway).

Since we now have multiple sources of link information (the
kernel for addresses and DHCP for routes and DNS, plus various
bits of static information), also move all the code that sets
mLinkProperties into one place. This allows us to be clearer
about who is authoritative for which parts of mLinkProperties.

Bug: 10232006
Change-Id: I4f262342379ce7fed172b67a1d403e3cd34f4232