I don't know anything about the legal and contractual aspects here (that stuff is mostly secret anyhow), but I would be shocked if they did not use Google's cod

I don't know anything about the legal and contractual aspects here (that stuff is mostly secret anyhow), but I would be shocked if they did not use Google's code to do this. To not do so would mean reimplementing Dalvik and all the Android infrastructure libraries. That's a huge, huge amount of code, and it would need to be bug-for-bug compatible with actual Android code. Instead, forking it is far more practical (if still hard).