The android client needs updating to support split-DNS:

• update code to use net/dns and new router APIs. May have to use wgengine/router.CallbackRouter. Bunch of APIs moved around, so need to get it back to building successfully again.
• Figure out how to read the "base" DNS config out of android, i.e. the config with no tailscale stuff in it. Might be as simple as reading /etc/resolv.conf from the app, if sandboxing hides tailscale's DNS settings from the app, or might be more complex.
• Test. Right now split DNS requires a server side feature flag to get split configs, so if I'm not the one that ends up testing it, may need to toggle the flag on extra domains.

cc @eliasnaur FYI