How to prevent an app from kill

[mahanmantare]

I am using an app of floating Back Button for my ease. That app uses accessibility feature to observe my action of course. But when ever i clear my recent apps, this app also killed. I would have to reopen it, give again permission of Accessibility to make it work.
Even if i dont kill the app at Recent app still system kill it after 1 hour.
Use different apps for back button but result is same. Any guide would be appreciated. Thanx in advance.. Waiting...