[Xamarin.Android.Build.Tasks] Update F# targets imports (#141)

* Make consistent with xamarin-ios
* Add modern `$(VisualStudioVersion)`-based import
* Add 4.0 import, remove 3.0 import

Author: mhutch

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

@@ -33,9 +33,20 @@ Copyright (C) 2012 Xamarin. All rights reserved.
         <GenerateSerializationAssemblies>Off</GenerateSerializationAssemblies>
         <_AndroidResourceDesigner>Resource.Designer.fs</_AndroidResourceDesigner>
     </PropertyGroup>
-    <Import Project="$(MSBuildExtensionsPath32)\..\Microsoft SDKs\F#\3.0\Framework\v4.0\Microsoft.FSharp.Targets" Condition="Exists('$(MSBuildExtensionsPath32)\..\Microsoft SDKs\F#\3.0\Framework\v4.0\Microsoft.FSharp.Targets') AND !Exists('$(MSBuildExtensionsPath32)\..\Microsoft SDKs\F#\3.1\Framework\v4.0\Microsoft.FSharp.Targets')" />
-    <Import Project="$(MSBuildExtensionsPath32)\..\Microsoft SDKs\F#\3.1\Framework\v4.0\Microsoft.FSharp.Targets" Condition="Exists('$(MSBuildExtensionsPath32)\..\Microsoft SDKs\F#\3.1\Framework\v4.0\Microsoft.FSharp.Targets')" />
+
+    <!-- xbuild searches multiple MSBuildExtensionsPath32, but only in the Import element, so we can't determine this with a variable -->
+    <Import
+      Condition="'$(Language)' == '' And Exists('$(MSBuildExtensionsPath32)\Microsoft\VisualStudio\v$(VisualStudioVersion)\FSharp\Microsoft.FSharp.Targets')"
+      Project="$(MSBuildExtensionsPath32)\Microsoft\VisualStudio\v$(VisualStudioVersion)\FSharp\Microsoft.FSharp.Targets" />
+    <Import
+      Condition="'$(Language)' == '' And Exists('$(MSBuildExtensionsPath32)\..\Microsoft SDKs\F#\4.0\Framework\v4.0\Microsoft.FSharp.Targets')"
+      Project="$(MSBuildExtensionsPath32)\..\Microsoft SDKs\F#\4.0\Framework\v4.0\Microsoft.FSharp.Targets" />
+    <Import
+      Condition="'$(Language)' == '' And Exists('$(MSBuildExtensionsPath32)\..\Microsoft SDKs\F#\3.1\Framework\v4.0\Microsoft.FSharp.Targets')"
+      Project="$(MSBuildExtensionsPath32)\..\Microsoft SDKs\F#\3.1\Framework\v4.0\Microsoft.FSharp.Targets" />
+
     <Import Project="Xamarin.Android.Common.targets" />
+
     <!--
     *******************************************
       Extensibility hook that allows VS to