Add mono.android reference Xamarin.Forms

1. In solution explorer, right click on References.
2. Click on Add Reference. 
3. Click on Browse on the left panel. 
4. Click on Browse in the bottom.
5. Enter this path: C:\Program Files (x86)\Microsoft VisualStudio\2017\Community\Common7\IDE\ReferenceAssemblies\Microsoft\Framework\MonoAndroid\v8.1.
6. Select Mono.Android.dll.
7. Check and add to the project.