I worked on something like this long time ago. It was not related to automotive but mobile industry. At that time it was difficult to get any such information. I don't know whether things have changed now, but I will give you my approach to understanding the low-level android stack. I approached it from bottom-up level rather than top-down. I looked at the hardware driver(I was working on enabling graphics stack for the devices, so I started digging into the graphics driver and its related userland libraries). Then I looked at the HAL layer that was related to the graphics stack. Then went through the skia library and the surface flinger, then the corresponding JNI calls. Most of the time you don't have to touch the JNI part of the stack.
For JNI please refer the documentation on Oracle's website.
For everything below that, the best approach is to just dive into the AOSP code(https://source.android.com). There is lots of documentation there as well. But the source code still gives you a lot better insight into the workings of the system and what the top layer expects from the bottom layer. As they say, "Read the source Luke!" :)
