[One .NET] exclude Xamarin.Android.PCLSupport.targets (dotnet#5430)

We are currently importing/using `Xamarin.Android.PCLSupport.targets`,
which does not appear to be needed at all in .NET 6.

Even if I add a PCL such as PCLStorage:

	<PackageReference Include="PCLStorage" Version="1.0.2" />

The `ImplicitlyExpandDesignTimeFacades` target doesn't do any actual work:

	Task "GetDependsOnNETStandard" skipped, due to false condition; ('$(_HasReferenceToSystemRuntime)' != 'true' and '$(_IsXBuild)' != 'true' and '$(DependsOnNETStandard)' == '' and '@(_XACandidateNETStandardReferences->Count())' != '0') was evaluated as ('true' != 'true' and 'true' != 'true' and '' == '' and '156' != '0').
	Task "Message" skipped, due to false condition; ('%(ReferencePath.ResolvedFrom)' == 'ImplicitlyExpandDesignTimeFacades') was evaluated as ('{RawFileName}' == 'ImplicitlyExpandDesignTimeFacades').

The dotnet/sdk, seems to already know how to bring in
`netstandard.dll`, and we don't need to do anything here.

Let's exclude `Xamarin.Android.PCLSupport.targets` and
`Xamarin.Android.PCLSupport.props` from the .NET 6 packages and stop
importing them.

This should save a small amount of time on every .NET 6 build:

	4 ms  ImplicitlyExpandDesignTimeFacades          3 calls

This target was being called N+1 times per `$(RuntimeIdentifier)` and
is completely removed now. This was a ~4ms savings on a "Hello World"
.NET 6 application on macOS.

build-tools/installers/create-installers.targets

@@ -257,8 +257,8 @@
     <_MSBuildFiles Include="$(MSBuildSrcDir)\Xamarin.Android.EmbeddedResource.targets" />
     <_MSBuildFiles Include="$(MSBuildSrcDir)\Xamarin.Android.FSharp.targets" ExcludeFromAndroidNETSdk="true" />
     <_MSBuildFiles Include="$(MSBuildSrcDir)\Xamarin.Android.Legacy.targets" ExcludeFromAndroidNETSdk="true" />
-    <_MSBuildFiles Include="$(MSBuildSrcDir)\Xamarin.Android.PCLSupport.props" />
-    <_MSBuildFiles Include="$(MSBuildSrcDir)\Xamarin.Android.PCLSupport.targets" />
+    <_MSBuildFiles Include="$(MSBuildSrcDir)\Xamarin.Android.PCLSupport.props"                                    ExcludeFromAndroidNETSdk="true" />
+    <_MSBuildFiles Include="$(MSBuildSrcDir)\Xamarin.Android.PCLSupport.targets"                                  ExcludeFromAndroidNETSdk="true" />
     <_MSBuildFiles Include="$(MSBuildSrcDir)\Xamarin.Android.SkipCases.projitems" />
     <_MSBuildFiles Include="$(MSBuildSrcDir)\Xamarin.Android.Tooling.targets" />
     <_MSBuildFiles Include="$(MSBuildSrcDir)\Xamarin.Android.Tools.Aidl.dll" />

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

@@ -2615,7 +2615,8 @@ because xbuild doesn't support framework reference assemblies.
 
 <Import Project="$(MSBuildThisFileDirectory)Xamarin.Android.Application.targets"
         Condition=" '$(AndroidApplication)' == 'True' "/>
-<Import Project="$(MSBuildThisFileDirectory)Xamarin.Android.PCLSupport.targets" />
+<Import Project="$(MSBuildThisFileDirectory)Xamarin.Android.PCLSupport.targets"
+        Condition=" '$(UsingAndroidNETSdk)' != 'true' " />
 <Import Project="$(MSBuildThisFileDirectory)Xamarin.Android.Designer.targets" />
 
 <Import Project="$(MSBuildThisFileDirectory)Xamarin.Installer.Common.targets"