Avoid exiting app with a single tap on the "back" button

While testing the app, I have seen that it's too easy to completely exit it. For example, I open the "customer" selection, for some reason I want to go back. The app provides a "Schlissen" (sorry, no German chars on my keybaord, so I use "ss") button on the top right that allows me to go back, but my brain is hardwired to the "back" button on the bottom android standard menu, so I click that button instead of the "Schlissen" one, and I close the whole app by mistake.

In my opinion, the android "back" button should work as the "Schlissen" button.

Also, it should be a nice addition to have to double tap the back button twice rapidly to actually exit the app. This is to avoid exiting by mistake (and having to re-enter all of your selections).

PS: "Schlissen" button is not translated to english!