Expose captive portal urls for configuration

Carriers in Mainland China need to customize certain captive portal
urls. The main issue is that google servers are not accessible in
Mainland China.

Added the following captive portal resources to be targeted for overlay.
- config_captive_portal_http_url
- config_captive_portal_https_url
- config_captive_portal_fallback_urls (string-array)
- config_captive_portal_fallback_probe_specs (string-array)

These values can be customized for e g diffent countries

Bug: 111819230
Test: atest FrameworksNetTests NetworkStackTests
Test: Add a product RRO that targets a specific country code,
insert a SIM card that matches that country code and check the log
what URL is used.
Change-Id: I38a294a5e14417f65e96e76235ea153c53c4d7cc