AOSP is a separate initiative from Google to release the source code of Android to the public on a regular basis. Development of Android happens almost entirely in-house and contributions/requests from the general public are treated with minimum priority. The public AOSP repo is only updated with these changes after each new release of Android is finished. Google is not forced to release the source code, aside from their changes to the Linux kernel, which are minimal anyway as Android is moving closer to mainline. They may have separate agreements with OEMs that guarantee source code access but that's of no concern to Epic Games as a user/developer on the platform and not an OEM.