You'll have a hell of a job pulling that one off as libnativehelper wasn't introduced until jellybean
https://android.googlesource.com/platform/libnativehelper/+refs . Which probably also explains why it doesn't exist as a prebuilt.
Obviously the functionality you require is going to be implemented elsewhere ( hopefully ) . I can highly recommend
http://androidxref.com/ if you need to do some code hunting.
To answer your original question. I'd try and grab the bare minimum required by hacking editing the .repo/manifest.xml to remove all the unnecessary fluff. Also there is a mirror of the AOSP on github, github allows you to download archives ( zips ) of a single branch which is always an option should you find yourself needed to take a more piecemeal approach.