How to Integrate another Android project into Unity project?

I'm currently trying to implement some code to make use of the Google XAPK expansion files they have introduced. The Google Play Downloader Library that Google supplies is it's own Android project with it's own resources.

The Java code I'm writing in Unity makes use of this library. This isn't a problem as I can just export the whole library project in .jar file and place it in \Assets\Plugins\Android\libs. This allows me to use it but whenever the library tries to access it's own resources then it fails as it can't find them. I've made sure to include the whole library project when exporting the .jar and not just the \src files.

Can this be done? If it can't does that effectively mean that we can't use the Google Play Libraries for downloading the XAPK files?