Try the passed in string without an extension on Android for resource loading

On iOS, you have to pass the extension to Splat in order to load an image via resource:

```cs
var img = BitmapLoader.Current.LoadFromResource("DefaultAvatar.png");
```

However, on Android, the generated Resource class doesn't include the extensions (rightfully so!), so the same code dies with a "Resource not found", even if you named it the same. With this PR, you can now name resources the same way and we'll guess to try to remove the extension