> Lastly, I haven't tried it, but I suspect replacing the Android Launcher would also let you do it. This is the method Facebook used for putting their talking heads in every app.
The talking heads overlay doesn't require replacing the Launcher. Installing just the FB Messenger app will get you that feature too.
AFAIK, it uses the "Draw over other apps" permission (android.permission.SYSTEM_ALERT_WINDOW) to achieve this. Here's a library that does the same: https://github.com/pingpongboss/StandOut/
The talking heads overlay doesn't require replacing the Launcher. Installing just the FB Messenger app will get you that feature too.
AFAIK, it uses the "Draw over other apps" permission (android.permission.SYSTEM_ALERT_WINDOW) to achieve this. Here's a library that does the same: https://github.com/pingpongboss/StandOut/