appdomain: relax netlink_socket neverallow rule

Relax the neverallow netlink restrictions for app domains.
In particular, some non-AOSP app domains may use netlink sockets
to communicate with a kernel driver.

Continue to neverallow generic netlink sockets for untrusted_app.
The intention here is that only app domains which explicitly need
this functionality should be able to request it.

This change does not add or remove any SELinux rules. Rather, it
just changes SELinux compile time assertions, as well as allowing
this behavior in CTS.

Modify other neverallow rules to use "domain" instead of "self".
Apps shouldn't be able to handle netlink sockets, even those
created in other SELinux domains.

Change-Id: I40de0ae28134ce71e808e5ef4a39779b71897571