[Xamarin.Android.Build.Tasks] _CreateAapt2VersionCache slow due to …

…wildcard usage (#2124)

Fixes: #2121

There were a number of performance issues with the
`_CreateAapt2VersionCache` target.

Firstly the wildcards were processing the *entire*
`$(IntermediateOutputPath)` tree!  What they should have been doing
was targeting specific directories, e.g. the root of
`$(IntermediateOutputPath)` and the directories under
`$(IntermediateOutputPath)\lp`.

Secondly the target did not have a `Condition` to stop it from
running if the versions matched, so that has been added.

Thirdly, even if a target is NOT run, MSBuild will still evaluate the
`<PropertyGroup/>`s and `<ItemGroup/>`s within the Target, so we need
to add a `Conditon` on the `@(_CompiledFlataArchive)` and
`@(_CompiledFlataStamp)` items as well to prevent evaluation and
corresponding directory traversal.

With these in place the time when this target is reduced to 1ms.

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

@@ -785,7 +785,7 @@ Copyright (C) 2011-2012 Xamarin. All rights reserved.
 <Target Name="_CreateAapt2VersionCache"
 		DependsOnTargets="_ReadAapt2VersionCache"
 		AfterTargets="_SetLatestTargetFrameworkVersion"
-		Condition="'$(_AndroidUseAapt2)' == 'True'"
+		Condition="'$(_AndroidUseAapt2)' == 'True' And '$(_Aapt2Version)' != '@(_Aapt2VersionCache)'"
 	>
 	<MakeDir Directories="$(IntermediateOutputPath)" Condition="!Exists('$(IntermediateOutputPath)')" />
 	<WriteLinesToFile
@@ -794,9 +794,10 @@ Copyright (C) 2011-2012 Xamarin. All rights reserved.
 			Lines="$(_Aapt2Version)"
 			Overwrite="true"
 	/>
-	<ItemGroup>
-		<_CompiledFlataArchive Include="$(IntermediateOutputPath)\**\*.flata"/>
-		<_CompiledFlataStamp Include="$(IntermediateOutputPath)\**\compiled.stamp"/>
+	<ItemGroup Condition="'$(_Aapt2Version)' != '@(_Aapt2VersionCache)'">
+		<_CompiledFlataArchive Include="$(_AndroidLibrayProjectIntermediatePath)**\*.flata" />
+		<_CompiledFlataArchive Include="$(IntermediateOutputPath)\*.flata" />
+		<_CompiledFlataStamp Include="$(_AndroidLibrayProjectIntermediatePath)**\compiled.stamp" />
 	</ItemGroup>
 	<Delete Files="@(_CompiledFlataArchive);@(_CompiledFlataStamp)" Condition="'$(_Aapt2Version)' != '@(_Aapt2VersionCache)'" />
 </Target>