It should be possible to add a native library (VS Build Action: AndroidNativeLibrary) to an "Android Class Library" project.
You can do this with MfA 4.2 but the library can't be found by P/Invoke when doing this. So, currently the native library must be located in executable/main project.
It is now supported in 4.6 and 4.7.x (AndroidNativeLibrary in 4.6, plus EmbeddedNativeLibrary in 4.7.x).
This doesn't work on 4.6.08007 (0cc7ae3b). The build action is there but the library can't be found by P/Invoke. Only works if the native library is included in the main project.
You need to use EmbeddedNativeLibrary in library project if you do not want to include it in the main project. If you do understand that and still believe it doesn't work, please provide a reproducible project that "should" work.