[Xamarin.Android.Build.Tasks] ConfigChanges.SmallestScreenSize mapping (#1242)

Fixes: https://github.com/xamarin/xamarin-android/issues/1234

The following user code:

	[Activity (ConfigurationChanges=ConfigChanges.SmallestScreenSize)]
	partial class MyActivity : Activity {
	}

was effectively a no-op, as the check for
`ConfigChanges.SmallestScreenSize` was instead checking for
`ConfigChanges.ScreenSize`; `ConfigChanges.SmallestScreenSize` would
*never* be interpreted.

Turns out it was from [monodroid/c2831532][0], a long-standing issue.

[0]: https://github.com/xamarin/monodroid/commit/c28315322e

src/Xamarin.Android.Build.Tasks/Utilities/ManifestDocumentElement.cs

@@ -218,7 +218,7 @@ namespace Xamarin.Android.Manifest {
 				values.Add ("navigation");
 			if ((value & ConfigChanges.Orientation) == ConfigChanges.Orientation)
 				values.Add ("orientation");
-			if ((value & ConfigChanges.SmallestScreenSize) == ConfigChanges.ScreenSize)
+			if ((value & ConfigChanges.SmallestScreenSize) == ConfigChanges.SmallestScreenSize)
 				values.Add ("smallestScreenSize");
 			if ((value & ConfigChanges.ScreenLayout) == ConfigChanges.ScreenLayout)
 				values.Add ("screenLayout");