[Microsoft.Android.Sdk] fix %(AndroidSkip*) item metadata

[jonathanpeppers]

The CheckMetadataSkipItemsAreProcessedCorrectly test was failing
under a dotnet context due to item metadata missing, such as:

<AndroidSkipAddToPackage>True</AndroidSkipAddToPackage>
<AndroidSkipJavaStubGeneration>True</AndroidSkipJavaStubGeneration>
<AndroidSkipResourceExtraction>True</AndroidSkipResourceExtraction>

These metadata support AndroidX.Migration, and they are completely
missing from the item groups generated in
Microsoft.Android.Sdk.AssemblyResolution.targets.

After the <ProcessAssemblies/> MSBuild task is called, we should
call <AppendCustomMetadataToItemGroup/> to populate the missing
metadata.

The CheckMetadataSkipItemsAreProcessedCorrectly test now passes.

[dellis1972]

Should these metadata items also be on _ShrunkAssemblies Items as well? Under normal .net these items end up on @(_ReferencePath) and @(_ReferenceDependencyPaths) which will then cascade down all the item groups that are used.

I'm curious why _AddAndroidCustomMetaData is not working as intended in this case? It appears in the BuildOrder.targets, but does not seem to work?

[jonathanpeppers]

Should these metadata items also be on _ShrunkAssemblies Items as well?

Yeah, I should add this.

I'm curious why _AddAndroidCustomMetaData is not working as intended in this case? It appears in the BuildOrder.targets, but does not seem to work?

_AddAndroidCustomMetaData still works "before the build", like for _ResolveLibraryProjectImports.

After the build in "legacy" Xamarin.Android, @(_ReferencePath) is passed through to <ResolveAssemblies/>:

https://github.com/xamarin/xamarin-android/blob/946f269e656fb42fba98b2e3490737bef77e682b/src/Xamarin.Android.Build.Tasks/Xamarin.Android.Legacy.targets#L259-L273

But in the new world, we use the @(ResolvedFileToPublish) item group coming from the dotnet/sdk. So we are basically starting over with any metadata we need.

But the bright side is, we don't call <ResolveAssemblies/> anymore!