[Thread] add config_thread_country_code_enabled to overlayable.xml

This commit adds `config_thread_country_code_enabled` to the
overlayable.xml to allow the device to overlay this configuration.

Bug: b/402276117
Test: Set the `config_thread_country_code_enabled` to `false` in
      the real Android device, then run the command
      `adb shell dumpsys thread_network` to check the state of
      CountryCode module.

Change-Id: I37cea57a38f5487c3a07508854adb6cfd25d50b2
diff --git a/service/ServiceConnectivityResources/res/values/overlayable.xml b/service/ServiceConnectivityResources/res/values/overlayable.xml
index f6dbf6c..28b46c1 100644
--- a/service/ServiceConnectivityResources/res/values/overlayable.xml
+++ b/service/ServiceConnectivityResources/res/values/overlayable.xml
@@ -49,6 +49,7 @@
             <!-- 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_country_code_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" />