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/property_contexts b/private/property_contexts
index 8860024..9031b28 100644
--- a/private/property_contexts
+++ b/private/property_contexts
@@ -1624,3 +1624,5 @@
 # Properties for app archiving
 pm.archiving.enabled      u:object_r:pm_archiving_enabled_prop:s0 exact bool
 
+# Properties for ThreadNetworkService
+threadnetwork.country_code u:object_r:threadnetwork_config_prop:s0 exact string