The kernel. Everything else from libc on up was rewritten, and under Google's control. The Play store is hardly in AOSP, much less their new IntelliJ-based development stack.

The tools are publicly developed: https://android.googlesource.com/platform/tools/build (there are also two adt repos, one for eclipse, one for idea; gerrit is also public)

Google may develop a lot of things behind closed doors but let's give them credit where credit's due.

The Play Store along with all the other Google Experience apps like gMail and Maps is not in AOSP. Google has no obligation to open-source those, nor do they want to.