Add DNS warning message

When private DNS (DNS over TLS) is active,
that setting a DNS for a static IP configuration will have no effect.

Add DNS warning message as below screenshot.
https://screenshot.googleplex.com/8JHK328tX6HxrYZ

Bug: 157620236
Test: Manual verify by WiFi Settings UI
Change-Id: Ibed34ceda32670f481d84b85b431d855af478dc3