Add sepolicy for the Thread Network property

This CL adds sepolicy for the system property
threadnetwork.country_code. This system property
is set by init and be read by the ThreadNetworkService.

Bug: b/309357909
Test: Configure the system property in ini.product.rc and
      check the configured country code via the command
      `dumpsys thread_network`.

Change-Id: I6f067ced24842755f2c5519169ba9a94df17829f
diff --git a/private/system_server.te b/private/system_server.te
index decabe1..6ba7d4a 100644
--- a/private/system_server.te
+++ b/private/system_server.te
@@ -1565,3 +1565,14 @@
 # Allow GameManagerService to read and write persist.graphics.game_default_frame_rate.enabled
 set_prop(system_server, game_manager_config_prop)
 
+# ThreadNetworkService reads Thread Network properties
+get_prop(system_server, threadnetwork_config_prop)
+
+# Do not allow any domain other than init and system server to set the property
+neverallow {
+  domain
+  -init
+  -vendor_init
+  -dumpstate
+  -system_server
+} threadnetwork_config_prop:file no_rw_file_perms;