[Xamarin.Android.Build.Tasks] Add support for '<AndroidNamespaceReplacement>'.

[jpobst]

Context: dotnet/java-interop#727

Add an MSBuild equivalent to the <ns-replace> metadata feature added in dotnet/java-interop#727.

This takes namespace transforms like:

<ItemGroup>
  <AndroidNamespaceReplacement Include='Androidx' Replacement='AndroidX' />
  <AndroidNamespaceReplacement Include='Com' Replacement='' />
  <AndroidNamespaceReplacement Include='Com.Google.' Replacement='Google' />
</ItemGroup>

And places them into a generated msbuild-metadata.xml file that is passed to generator:

<metadata>
  <ns-replace source='Androidx' replacement='AndroidX' />
  <ns-replace source='Com' replacement='' />
  <ns-replace source='Com.Google.' replacement='Google' />
</metadata>

Today, this only supports <ns-replace> metadata, but it could be expanded in the future if we feel that allowing other metadata is desirable. (I am personally skeptical 😁.)

[jonpryor]

The RemoveEventHandlerResolution failures are fixed by 599fffd.

[jonpryor]

I'm not sure you need the XDocument at all? XElement.WriteTo() also works…

[jpobst]

Updated.

[jonpryor]

@jpobst: the new item group should also be documented in: https://github.com/xamarin/xamarin-android/blob/main/Documentation/guides/building-apps/build-items.md

[jonathanpeppers]

Should we emit a warning/error if %(Replacement) is blank? I'm just thinking if someone put Replace="AndroidX" on accident, they won't know what's wrong?

[jpobst]

%(Replacement) being blank is valid and useful. I guess the question is should we require users to provide the blank attribute Replacement=""? Perhaps we should?

[jpobst]

Added and documented new error.

[jonathanpeppers]

This is probably fine, but feels weird. Would this be just as good?

Suggested change

	if (NamespaceTransforms?.Any () == true) {
	if (NamespaceTransforms != null && NamespaceTransforms.Length > 0) {

[jpobst]

Yes, those are equivalent. I think Any and null-check operator is generally considered the modern "best practice".

[jonathanpeppers]

I generally avoid using System.Linq;, but there are some cases where OrderBy() is ok.

This is fine, though.