Drop support for updating IPv6 addresses in clatd.

Currently, when clatd detects that the IPv6 address on its
interface has changed, it updates its configuration accordingly.
This complicates the code, and it prevents us from determining
the IPv6 address in netd instead of clatd.

Stop doing this, and simply exit instead. The framework will
notice that the interface is gone and restart clatd, which will
then have the correct configuration (and, unlike current code,
the correct NAT64 prefix, if that has changed as well).

This change also allows us to drop CAP_NET_ADMIN and run clatd
as an unprivileged user with no capabilities.

Test: atest clatd_test
Test: changing IPv6 address on interface causes clatd to exit
Test: "adb shell cat /proc/$(adb shell pidof clatd)/status | grep Cap"
Change-Id: I296dcb185f603f685223b24ffefdf7a5c22b66e6