Android: launching connection with a tap on screen/screensaver

I’m setting up a dedicated video chat tablet for a disabled person, whose input I want to be only a tap on the screen launching a connection request (to one user), ideally while a screensaver is running with family pics, with worst case UI being an icon button that fills the screen. I.e. a dementia-proof device. It seems there would be some kind of startup script-like method like this already available in Jami? Nothing came up in a search, seems it could be system-specific.

If the user has >1 possible person, on tap the screen shows a matrix of faces with a green Call button under each, which calls the API… it seems the glue I’m talking about would be a plugin? But not if it needs to be user-enabled after a call starts: “A media handler functionality only takes place if you turn them on during a call.”