Describe the problem that needs solving

It would be nice to have an option to disable closing the app by pressing back button.
I often accidentally close the app thinking that I am exiting fullscreen image/video preview.

Describe the solution you'd like

I am thinking about 2 options:

1. Option in the setting to disable closing the app by pressing back button.
2. Notification "Tap again to exit" at the bottom which saying that the next pressing back button will close the app.