The ScreenRetainer app is designed to lock the screen on a user-selected application.

How it works: you enter the application and you will be shown a list of installed applications on your device, in which you must select the application to commit. Once the selected app is open, it will work as usual, but if you try to exit that app, the device screen will be locked. This can be used when you need to put your phone in the hands of another person, or if you might fall asleep with your phone in your hands to prevent intruders from gaining full access to your device.

The app uses the Accessibility API for its operation