Use LinkAddress in address notifications.

Currently address{Updated,Removed} pass in the address as a
string such as "fe80::1/64". Use LinkAddresses instead, since
that's what it is.

This makes the code more robust in the unlikely case that netd
passes in an invalid string. In the future we can move flags and
scope into the LinkAddress itself and simplify the code further.

Bug: 9180552
Change-Id: I66599f9529cf421caa7676fdd0141bb110b8589e