Add overlayable tag for NetworkStackConfig

<?xml version="1.0" encoding="utf-8" ?>

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<resources xmlns:android="http://schemas.android.com/apk/res/android">

    <overlayable name="NetworkStackConfig">

        <policy type="product|system|vendor">

            <!-- Configuration values for NetworkMonitor -->

            <item type="string" name="config_captive_portal_http_url"/>

            <item type="string" name="config_captive_portal_https_url"/>

            <item type="array" name="config_captive_portal_fallback_urls"/>

        </policy>

    </overlayable>

</resources>