cf: add netdomain attribute to socket_forward_proxy

netdomain is the set of domains with network access. Adding
socket_forward_proxy to that set resolves this denial:

avc: denied { write } for comm="socket_forward_" laddr=127.0.0.1
lport=38980 faddr=127.0.0.1 fport=5555
scontext=u:r:socket_forward_proxy:s0
tcontext=u:r:socket_forward_proxy:s0 tclass=tcp_socket permissive=1

Bug: 28053261
Test: boot socket_forward_proxy without above denial
Change-Id: I469f672f95cc7df800b5f15cbe6c6c242afcf9f7
diff --git a/shared/sepolicy/socket_forward_proxy.te b/shared/sepolicy/socket_forward_proxy.te
index e7b9f30..00ffba7 100644
--- a/shared/sepolicy/socket_forward_proxy.te
+++ b/shared/sepolicy/socket_forward_proxy.te
@@ -1,4 +1,4 @@
-type socket_forward_proxy, domain;
+type socket_forward_proxy, domain, netdomain;
 type socket_forward_proxy_exec, exec_type, vendor_file_type, file_type;
 
 init_daemon_domain(socket_forward_proxy)