[Xamarin.Android.Build.Tasks] don't set $(PublishSelfContained) for .NET 9

[jonathanpeppers]

Context: dotnet/sdk@d21e6bf
Context: 8fa5d99
Context: 0cd963a

Building a net8.0-android project with a .NET 9 SDK will fail when using specific arguments:

> dotnet publish -r android-arm64
MSBuild version 17.9.0-preview-23577-01+31108edc1 for .NET
...
bin\Debug\dotnet\sdk\9.0.100-alpha.1.23603.1\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.Sdk.FrameworkReferenceResolution.targets(494,5): error NETSDK1112: The runtime pack for Microsoft.Android.Runtime.34.android-arm64 was not downloaded. Try running a NuGet restore with the RuntimeIdentifier 'android-arm64'.

Our repo also sets $(DisableTransitiveFrameworkReferenceDownloads), which causes a slightly different error message:

error NETSDK1185: The Runtime Pack for FrameworkReference 'Microsoft.Android.Runtime.34.android-arm' was not available. This may be because DisableTransitiveFrameworkReferenceDownloads was set to true.

In .NET 8, 0cd963a began setting $(PublishSelfContained) to false as a way to avoid the error:

Microsoft.NET.RuntimeIdentifierInference.targets(212,5): error NETSDK1191: A runtime identifier for the property 'SelfContained' couldn't be inferred. Specify a rid explicitly.

Because Android is always self-contained, we set $(SelfContained) through other means, and everything worked.

In .NET 9, other changes were made to where this "workaround" no longer works. To fix this, Rolf introduced dotnet/sdk#d21e6bf5 and $(AllowSelfContainedWithoutRuntimeIdentifier) as a way to fix the issue going forward in .NET 9. We are using the new property in .NET 9 since 8fa5d99.

Unfortunately, this leaves one case that is still broken:

• Building with .NET 9 SDK

• Building a net8.0-android project

• dotnet publish -r android-arm64

Which seems like for now, we could just condition our workaround for .NET 8 and below:

<PublishSelfContained Condition=" '$(PublishSelfContained)' == '' and $([MSBuild]::VersionLessThan($(NETCoreSdkVersion), '9.0')) ">false</PublishSelfContained>

Making this change locally, both cases work for me:

• .NET 8 project, .NET 8 SDK, dotnet publish -r android-arm64

• .NET 8 project, .NET 9 SDK, dotnet publish -r android-arm64

The only other solution would be to create our own empty _CheckForUnsupportedAppHostUsage target to avoid errors, but that seems explicitly worse. We should also backport dotnet/sdk#d21e6bf5 to .NET 8 SDKs, but it will probably take a month for that to ship.

After this is merged and we have a new, nightly .NET 8 build, we can restore the following unit test in main:

https://github.com/xamarin/xamarin-android/blob/40cc8eaf78eb9f95abcc0e966ab274cef1692acc/src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/XASdkTests.cs#L202-L205

[jonathanpeppers]

They are backporting: dotnet/sdk#37368

But this won't ship until sometime in the new year.

[jonpryor]

Draft commit message:

Context: https://github.com/dotnet/sdk/commit/d21e6bf5c5a239892d771f409841ce856bddcd89
Context: 8fa5d99341b868795b54bfa9d087bda38151a4d2
Context: 0cd963ac03fbd3d7370221adbc65b099e18f6565
Context: 95946d2e7f745ecc93efcd57cb862ae5d0912618
Context: 964746e83a85926b93057559dac70844b2ab3055

Building a `net8.0-android` project with a .NET 9 SDK will fail when
providing an RID:

	> dotnet publish
	# works

	> dotnet publish -r android-arm64
	MSBuild version 17.9.0-preview-23577-01+31108edc1 for .NET
	…
	bin\Debug\dotnet\sdk\9.0.100-alpha.1.23603.1\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.Sdk.FrameworkReferenceResolution.targets(494,5):
	error NETSDK1112: The runtime pack for Microsoft.Android.Runtime.34.android-arm64 was not downloaded. Try running a NuGet restore with the RuntimeIdentifier 'android-arm64'.

Our repo also sets `$(DisableTransitiveFrameworkReferenceDownloads)`
within unit tests (95946d2e), which causes a slightly different error
message:

	error NETSDK1185: The Runtime Pack for FrameworkReference 'Microsoft.Android.Runtime.34.android-arm' was not available. This may be because DisableTransitiveFrameworkReferenceDownloads was set to true.

which matches the TODO within 8fa5d993:

>  * A specific case of `dotnet publish -f net8.0-android -r android-arm`
>    is failing with:
> 
>        error NETSDK1185: The Runtime Pack for FrameworkReference 'Microsoft.Android.Runtime.34.android-arm' was not available. This may be because DisableTransitiveFrameworkReferenceDownloads was set to true.
> 
>    This is related to `$(AllowSelfContainedWithoutRuntimeIdentifier)`,
>    but I will address in a future PR.  This will likely require .NET 8
>    Android workload changes.

In .NET 8, 0cd963ac0 began setting `$(PublishSelfContained)`=false
as a way to avoid the error:

	Microsoft.NET.RuntimeIdentifierInference.targets(212,5):
	error NETSDK1191: A runtime identifier for the property 'SelfContained' couldn't be inferred. Specify a rid explicitly.

Because Android is always self-contained, we set `$(SelfContained)`
through other means (964746e8; `$(SelfContained)` is set as part of the
inner builds), and everything worked.

In .NET 9, other changes were made such that this "workaround" no
longer works.  To fix this, @@rolfbjarne introduced dotnet/sdk@d21e6bf5
and `$(AllowSelfContainedWithoutRuntimeIdentifier)` as a way to fix the
issue going forward in .NET 9.  We are using the new property in .NET 9
since 8fa5d993.

Unfortunately, this leaves one case that is still broken:

 1. Create a new project with `$(TargetFramework)`=net8.0-android
    (i.e. a .NET 8 project)

 2. Using the .NET 9 SDK

 3. Building while specifying a RID, e.g.
 
        dotnet publish -r android-arm64

It seems like, for now, we can just condition our workaround for
.NET 8 and below:

	<PublishSelfContained Condition=" '$(PublishSelfContained)' == '' and $([MSBuild]::VersionLessThan($(NETCoreSdkVersion), '9.0')) ">false</PublishSelfContained>

Making this change locally, both cases work for me:

  * .NET 8 project, .NET 8 SDK: `dotnet publish -r android-arm64`

  * .NET 8 project, .NET 9 SDK: `dotnet publish -r android-arm64`

The only other solution would be to create our own empty
`_CheckForUnsupportedAppHostUsage` target to avoid errors, but that
seems explicitly worse.  We should also backport dotnet/sdk#d21e6bf5
to .NET 8 SDKs, but it will probably take a month for that to ship.

After this is merged and we have a new, nightly .NET 8 build, we can
restore the [`XASdkTests.DotNetPublish()` unit test][0] on main.

[0]: https://github.com/xamarin/xamarin-android/blob/40cc8eaf78eb9f95abcc0e966ab274cef1692acc/src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/XASdkTests.cs#L202-L205