Parse IP address changes in NetlinkEvent.

This adds support for parsing RTM_NEWADDR and RTM_DELADDR
netlink messages received on netd's netlink socket and
formatting them them so NetlinkHandler can process them.

Address changes are notified in subsystem "address". Interface
changes, which used to be notified in subsystem "net", are now
notified in subsystem "interface" to avoid confusion.

[Cherry-pick of a4b4e9ad]

Bug: 10232006
Change-Id: I93a50e8de17014e118a42f5cc0eb90a0cbfa73cc