One option I like is the app "Conversations" (available on F-Droid and Play Store). It's a nice XMPP client with Signal-based (OMEMO) encryption, a great UI (in

One option I like is the app "Conversations" (available on F-Droid and Play Store). It's a nice XMPP client with Signal-based (OMEMO) encryption, a great UI (in my opinion anyway), and it's extremely battery friendly in my experience -- my device doesn't have GCM/FCM. You'll need an XMPP server though, and to use the E2E encryption you'll need a server which supports certain XEPs. The app dev also runs a server at conversations.im that is, of course, compatible. It costs €8/yr though. That said, there are several public servers with OMEMO support, plus self-hosting is an option.