Properly set the property_context for net.tcp_init_rwnd.

This property is many years old and it does not have a property
context associated with it. It is set by the system server (in
particular, ConnectivityService code, in the Tethering module)
and read by init, which does:

on property:net.tcp_def_init_rwnd=*
    write /proc/sys/net/ipv4/tcp_default_init_rwnd ${net.tcp_def_init_rwnd}

There is no need to add read access to init because init can read
and write any property.

Test: m
Fix: 170917042
Change-Id: I594b09656a094cd2ef3e4fd9703e46bf7b2edd4c
diff --git a/private/system_server.te b/private/system_server.te
index e1f97e9..4f302b2 100644
--- a/private/system_server.te
+++ b/private/system_server.te
@@ -636,6 +636,7 @@
 set_prop(system_server, safemode_prop)
 set_prop(system_server, theme_prop)
 set_prop(system_server, dhcp_prop)
+set_prop(system_server, net_connectivity_prop)
 set_prop(system_server, net_radio_prop)
 set_prop(system_server, net_dns_prop)
 set_prop(system_server, usb_control_prop)