Populate 464xlat LinkProperties only when the interface comes up.

Currently Nat464Xlat reads the clat IPv4 address and updates the
clat LinkProperties when the interface is created. This causes a
race condition: because clatd only sets the IPv4 address after
creating the interface, it's possible that Nat464Xlat will read
the address before clatd has set it, causing the framework to
think that the clat IPv4 address is 0.0.0.0/32.

This seems to be happening more frequently now, perhaps because
clatd takes a bit longer to configure the IPv4 address now that
it needs to check that the address is free before using it.

Fix this by making Nat464Xlat listen for the interface coming up
instead of listening for the interface being added.

Bug: 12111730
Change-Id: Ic1c59b5b6dbb851b7431d1b06885f67803373bb9