Use /dev/socket/dnsproxyd on Android rather than 8.8.8.8 for MX record resolution

[link2xt]

Currently the core falls back to using 8.8.8.8 resolver if /etc/resolv.conf is not found, which is the case on Android:

deltachat-core-rust/src/provider.rs

Lines 163 to 177 in 2f5c6b5

	/// Get resolver to query MX records.
	///
	/// We first try to read the system's resolver from `/etc/resolv.conf`.
	/// This does not work at least on some Androids, therefore we fallback
	/// to the default `ResolverConfig` which uses eg. to google's `8.8.8.8` or `8.8.4.4`.
	fn get_resolver() -> Result<TokioAsyncResolver> {
	if let Ok(resolver) = AsyncResolver::tokio_from_system_conf() {
	return Ok(resolver);
	}
	let resolver = AsyncResolver::tokio(
	config::ResolverConfig::default(),
	config::ResolverOpts::default(),
	);
	Ok(resolver)
	}

This fallback was added in #2852

We should fallback to using /dev/socket/dnsproxyd instead which is a stable interface maintained even on newer Android for compatibility reason and it is easy to use: hickory-dns/hickory-dns#652 (comment)
Queries can be created using hickory-proto which is the same library we use for DNS resolution now: https://docs.rs/hickory-proto/latest/hickory_proto/op/query/struct.Query.html

Related forum thread: https://support.delta.chat/t/why-connection-to-googledns-8-8-8-8/2074

For Linux platform this /dev/socket/dnsproxyd can be tried unconditionally as deltachat-rpc-server binaries do not know if they are going to be running on Linux or in Termux on Android.