Check that DoH provider list is not used if DDR is enabled,

This helps ensure the code behaves correctly when DDR is enabled
and the device is using the Google and Cloudflare DNS servers
that are already supported because they are in the hardcoded
list of DoH providers.

Also add a bit more test coverage to resolv_private_dns_test.

Bug: 240259333
Test: test-only change
Change-Id: I77fffcc00270c3da5686f6509a2a8a3d8b687c7a