Building a chat app is no child app. It is a highly complex process that involves several critical decisions. Right from choice of programming language to hosting and secured storage, everything is of critical importance.
The most common ways to build a android chat app is through starting from scratch or development using SaaS solutions or choosing a readymade solution!
Visit: https://www.behance.net/gallery/41813795/Build-Android-Chat-Messenger-App
Unfortunately the chat messenger market is highly centralized. The most popular chat apps are closed-source so only the developer´s app is compatible. And since the number of users is the most critical factor, there is a huge inertia effect that makes very difficult for the new apps to grow. If the same happened to email, we will still have 50MB mailboxes.
There are great open-source instant message services such as XMPP and IRC but they were not able to grow enough when MSN started to fall. I hope in the next chat revolution an open-source protocol will get the biggest marketshare.