Android Question Adding App access from another App's screen

I apologize if this question has already been asked but my various forum searches have not revealed any helpful results.

I would like to add functionality to an App while another App has focus (i.e. displayed on the screen).

The procedure would be be:

1. Start my App (Added functionality) as a service
2. Detect when the other App has focus (i.e. displayed)
   
3. Display my graphics or button on the other App's display (The other App would be fully functioning and it's display has normal operation other than a small button or graphic overlaying the original display)
   
4. Display my own App when the user presses the button or graphic
5. ---- Perform action/function in my app
6. Return to the other App when the user presses the back button
7. Remove the button/graphic from the display when the other App looses focus

That is it in a simplistic nutshell. I would be very grateful if anyone could point me in the right direction.

Thanks,

John