As far as I can see it is currently impossible to have cross-platform assets stored in a shared project. This is because they require different build actions depending on the platform: "Content" in iOS, "AndroidAsset" in Android, and therefore cannot be reused between platforms.
Is there a reason why you introduced "AndroidAsset"? Could "Content" build action in Xamarin.Android projects be made to behave in the same way as "AndroidAsset"? This would allow creation of shared projects with assets.