[Thread] add missing Thread overlay decl config_thread_border_router_default_enabled

Bug: 401654864

Change-Id: I45886e094eddf078b36b23757fca17f650fb9420
diff --git a/service/ServiceConnectivityResources/res/values/overlayable.xml b/service/ServiceConnectivityResources/res/values/overlayable.xml
index 5c0ba78..f6dbf6c 100644
--- a/service/ServiceConnectivityResources/res/values/overlayable.xml
+++ b/service/ServiceConnectivityResources/res/values/overlayable.xml
@@ -48,6 +48,7 @@
 
             <!-- Configuration values for ThreadNetworkService -->
             <item type="bool" name="config_thread_default_enabled" />
+            <item type="bool" name="config_thread_border_router_default_enabled" />
             <item type="bool" name="config_thread_location_use_for_country_code_enabled" />
             <item type="string" name="config_thread_vendor_name" />
             <item type="string" name="config_thread_vendor_oui" />