[msbuild] setup inputs and outputs for XamlC target

Context: https://github.com/jonathanpeppers/XamarinAndroidBuildTimes

After doing a profiling/review of Xamarin.Android project build times, I
noticed that the `XamlC` target was running on every build no matter
what. The project I used was the Forms Master/Detail template from VS
2017 15.6.4.

In my test repo, I was timing the following situations:
1. Completely clean/fresh build
2. Build again, no changes
3. Change C#, build again
4. Change `AndroidResource` XML, build again

In all cases `XamlC` was running, due to the task not having setup
proper inputs and outputs for the MSBuild target. Thinking about it, it
seemed like we could skip `XamlC` as long as the input assembly did not
change, such as case no. 2 or no. 4.

Changes to the `XamlC` target:
- Setup `$(IntermediateOutputPath)$(TargetFileName)` (the assembly) as input
- Setup a `XamlC.stamp` file as an output
- `<Touch />` the `XamlC.stamp` file after running the `XamlCTask`
- Add to the `FileWrites` MSBuild item, so that the `IncrementalClean`
target doesn't delete the stamp file

On my Windows machine, this improved the following build times for cases:
1. same (XamlC should run)
2. 3.685s -> 2.887s
3. same (XamlC should run) (would also be same as XAML changing)
4. 12.126s -> 11.214s

Since this was basically an empty project, I suspect the improvements
would be more drastic for apps with lots of XAML and using `XamlC`.

Author: jonathanpeppers

.nuspec/Xamarin.Forms.targets

@@ -85,17 +85,17 @@
 		</CompileDependsOn>
 	</PropertyGroup>
 
-	<Target Name="XamlC" AfterTargets="AfterCompile" Condition="'$(_XamlCAlreadyExecuted)'!='true'">
-		<PropertyGroup>
-			<_XamlCAlreadyExecuted>true</_XamlCAlreadyExecuted>
-		</PropertyGroup>
+	<Target Name="XamlC" AfterTargets="AfterCompile" Inputs="$(IntermediateOutputPath)$(TargetFileName)" Outputs="$(IntermediateOutputPath)XamlC.stamp">
 		<XamlCTask
 			Assembly = "$(IntermediateOutputPath)$(TargetFileName)"
 			ReferencePath = "@(ReferencePath)"
 			OptimizeIL = "true"
 			DebugSymbols = "$(DebugSymbols)"
 			DebugType = "$(DebugType)"
 			KeepXamlResources = "$(XFKeepXamlResources)" />
+		<Touch Files="$(IntermediateOutputPath)XamlC.stamp" AlwaysCreate="True">
+			<Output TaskParameter="TouchedFiles" ItemName="FileWrites"/>
+		</Touch>
 	</Target>
 
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