tl;dr: not gonna happen
Nearly everything about Android the operating system. Android the app platform is deeply tied to Google Play Services, which is not open source and is once more deeply tied to Google's online services.
And they've re-routed apps' access to basic phone data like GPS, camera and microphone through Google Play Services, so that if you want to use any app that uses those services you also have to let Google log those data.
It's almost as bad as tivoization from the FOSS perspective, while also invading your privacy. Yay.
Edit: Every OEM would have their own spin, for differentiation. Lenovo too...
One can buy an oem or restricted region license for windows for 20$ but legally it's about the same as pirating it.
The question is how much third-party code there is in the NT kernel. There may even be remainders of the cooperation with IBM tainting the source code (Windows NT did have an OS/2 personality at some point).
I expect most of that stuff lives in the userland interfaces to the kernel, but at least Microsoft seems to consider those to still be kernel code.
People like to shit on Windows with claims that Linux does it better. Actually seeing the source and reading about the internals (anyone can do that last bit) let's you see in just how many places Linux was and still is playing catch up. Linux may have a better implementation for some use cases, but to claim architectural superiority is just crazy in my eyes.
Anyway, the license for the WRK is actually quite open. Seeing it doesn't prevent you from contributing to open source projects, you can share it with anyone in a university setting, can keep it on your personal devices, and can share snippets of source code in blog posts, papers and the like. If you are eligible, willing to comply with the license, and can find someone who still has it (the hardest of the requirements) it's fair game. In my efforts I asked Microsoft for permission as well, but I didn't get the sense that was necessary (but shouldn't hurt).
You can still build the WRK on modern Windows, or on the included Virtual PC Windows Server 2003 images. The output dll can be installed in place into compatible versions of Windows. The distribution includes a full OS course, the original internal design workbook (straight from Dave Cutler's hand, in the original .DOC!) and labs detailing how to modify/extend the most interesting part of the kernel.
Source: I'm working on a tool/resource to make OS dev more accessible to students at my university (as well as to myself) and made vigorous efforts to procure the Windows Kernel for comparative purposes.
Trusted partners that develop for windows and oems can also gain access.
I'd be amazed if they ever make it available under anything like a GPL or BSD style license.
This idea is probably more feasible than first glance would suggest, but still, I wouldn't hold your breath.