Improve SRV record resolving (resolves bitfireAT/davx5#18)

- Android 8 to 10: use all DNS servers; prioritize resolvers of active connections
  (before: use only DNS servers of "active" connection, which may be a VPN connection without servers)
- Android 10+: use new DnsResolver API so that we don't need to know DNS servers anymore (also supports DoT etc.)