What the fake android update screen is
Android system updates apply on a near-black screen with the green accent that has been the platform’s signature since the beginning. The wording varies a little between manufacturers, but the shape of the screen is consistent.
On a phone this is best run from a home-screen shortcut, which opens the page with no browser interface at all.
Android system update prank, no app needed
Add the page to your home screen from the browser menu, then open it from there. Android honours that as a standalone launch, so no address bar appears.
Nothing is installed and no permission is requested — in particular, this is not an app, and there is nothing to grant.
How to run it, and how to stop it
Press F for fullscreen, then F11 to hide the browser’s own toolbars. That second step is the one that matters: an address bar sitting above the screen gives the whole thing away instantly.
To end it: Escape leaves fullscreen, F11 brings the toolbars back, and closing the tab removes it entirely. Nothing is installed, no file is written and no setting is changed, so there is nothing to undo afterwards.
Worth saying plainly — use this on people who will find it funny, and on machines you are allowed to touch. Show them the way out once the joke has landed.