XEP-0380: Explicit Message Encryption

[daniele-athome]

https://xmpp.org/extensions/xep-0380.html

Using https://github.com/Flowdalic/Smack/blob/master/smack-experimental/src/main/java/org/jivesoftware/smackx/eme/ExplicitMessageEncryptionManager.java

Also begin decoupling usage of E2EEncryption to prepare for new ways of encrypting messages.