Tutorial about layers from app to hal?

[Deep Impact]

Hi, I'm searching something like a tutorial which can show me how to introduce and implement my own Hardware Abstraction Layer in AOSP, and how to use in inside an app.
I have the following very simple use case, just for me to understand the whole communication/layers between App and HAL.

Let's assume, I've got an Hardware Button, which I want to simulate, which can be ON or OFF.
So just a boolean value as an information will work for me. The communication shall be bidirectional.
I want to toggle ON/OFF the button by app also.

Now, where do I start?

A simple tutorial which has a breakthrough all layers would be helpful for me.

thx!