[Xamarin.Android.Build.Tasks] skip _FindLayoutsToBind when feature is…

… unused

Context: xamarin#4187

We have been able to repro a hang when using ReSharper +
Xamarin.Android with this stack trace:

    Xamarin.Build.AsyncTask.dll!Xamarin.Build.AsyncTask.WaitForCompletion() Line 285
    Xamarin.Build.AsyncTask.dll!Xamarin.Build.AsyncTask.Execute() Line 226
    Xamarin.Android.Build.Tasks.dll!Xamarin.Android.Tasks.AndroidAsyncTask.RunTask()
    Xamarin.Android.Build.Tasks.dll!Xamarin.Android.Tasks.AndroidAsyncTask.Execute()

This is happening inside the `<CalculateLayoutCodeBehind/>` MSBuild
task.

Using ReSharper's special "new class" dialog, a design-time build
causes the UI to lock up in VS.

PR xamarin#4187 does not fully solve the problem, however the problem *does*
go away if I make `_FindLayoutsToBind` conditional.

`_FindLayoutsToBind` can be completely skipped if you aren't using the
new code-behind features in Xamarin.Android.

This is a good fix for now to cover most Xamarin.Android projects, and
we can revisit `<CalculateLayoutCodeBehind/>` at a later time.

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.targets

@@ -1462,7 +1462,8 @@ because xbuild doesn't support framework reference assemblies.
   </ItemGroup>
 </Target>
 
-<Target Name="_FindLayoutsForBinding" Condition=" '$(Language)' == 'C#' ">
+<Target Name="_FindLayoutsForBinding"
+    Condition=" '$(Language)' == 'C#' And ('$(AndroidGenerateLayoutBindings)' == 'True' Or '@(AndroidBoundLayout)' != '') ">
   <FindLayoutsToBind
       GenerateLayoutBindings="$(AndroidGenerateLayoutBindings)"
       BoundLayouts="@(AndroidBoundLayout)"