refer to this pages to see the issue
the fix is pretty easy:
1- remove this next 3 libs(and his dependencies)** from your Android project
Microsoft.Net.Http Microsoft.Bcl Microsoft.Bcl.Build
2- Add System.Net.Http reference Manually via "References/All/System.Net.Http"
now you're able to debug your app even using Shared Mono Runtime / Fast Assembly Development
**It suppose that Microsoft.Bcl, Bcl.Build and Microsoft.Net.Http and his dependencies are no mandatory to have it into you Android App Project it will work by adding them only for your PCL
if you have any doubt dont forget that you can contact me by email on firstname.lastname@example.org