Cronvoy: support dynamic DNS configuration

With Android, the User can change the DNS server IP address at will. And Cronet respects the User's choice: there is a 50ms loop polling the current Android DNS configuration to keep Cronet up-to-date. This feature is available since [Marshmallow (V6.0 / API level 23)](https://source.android.com/setup/start/build-numbers#platform-code-names-versions-api-levels-and-ndk-releases)

The logic to capture the current DNS configuration for Android must be written in Java. This is what [Cronet does](https://source.chromium.org/chromium/chromium/src/+/main:net/android/java/src/org/chromium/net/AndroidNetworkLibrary.java;l=405).