Change the way we use to block IPv6 #853

I am talking about pref.block.ipv6 low-level setting.

Relevant discussion:
https://forum.adguard.com/index.php?threads/email-not-syncing-since-upgrade-to-7.14862/

https://play.google.com/store/apps/details?id=de.lennartschoch.disableipv6&hl=en
http://forum.xda-developers.com/general/networking/guide-disable-ipv6-android-t3298659