Please see https://forums.bitfire.at/post/12828
Support TOR (.onion) urls through a socks proxy
An updated (v2) pull request for supporting connections to “http://xyz.onion” through tor. As the original patch, it allows you to host e.g. owncloud without the need for a dyndns service. Davdroid will use a socks proxy (on 127.0.0.1:9050) to connect to an .onion address.
Please see #407 and https://davdroid.bitfire.at/faq/entry/using-with-tor and review the necessity of this patch.