I had run into this issue when trying to build NuGet packages for Android projects before but had forgotten the solution.
The following thread details the resolution :-
https://forums.xamarin.com/discussion/48765/how-do-i-automatically-version-xamarin-android-library-projects
In summary, ensure the appropriate Mono.Android.dll version is copied to the \bin\Debug folder for your project and then the various details will be picked up from the AssemblyInfo.cs, including the Version number.
