Separate NetworkMonitor/framework portal URL

The framework cannot return URLs used by the updatable NetworkStack,
which may use configurable URLs, changing URLs, or mechanisms not
involving URLs to detect captive portals. NetworkMonitor has already
been using random fallback URLs for a while that do not match the value
returned by ConnectivityManager#getCaptivePortalServerUrl.

With this change, the default value returned by the framework is
configured in framework resources as
config_networkDefaultCaptivePortalServerUrl. NetworkMonitor behavior may
change as it is an updatable component, but the current URL is
configured in NetworkMonitor resources as
config_captive_portal_http_url.

Test: flashed, booted, WiFi and captive portal working
Test: ConnectivityManager#getCaptivePortalServerUrl returns correct
      value.
Bug: 127908503
Change-Id: I371dedc5b22efa909d7fd58e1ebe9b8aaced9780