Shortcut to quickly kill an app
I have always felt the need for this and been using in KitKat and Marshmallow. 

Since your device is rooted, it's easy. Install Gravity Box Xposed module and under Navigation bar tweaks &rarr; Custom key settings &rarr; Long press action (Kill foreground app)  and choose your Icon Style/Set custom image for the icon to sit always on your Navigation Bar. Simply press it to get rid of the unresponsive app

You see the icon on extreme left, below Yahoo mail