[Xamarin.Android.Build.Tasks] Add XA1031 error for AndroidHttpClientHandlerType

[dellis1972]

Fixes #7326

We should check the value of the AndroidHttpClientHandlerType property during the build process. This property can have a number of different values depending on what version of Xamarin.Android is being used.

Under Classic a valid value is Xamarin.Android.Net.AndroidClientHandler, however under .NET 6+ the valid values are

• Xamarin.Android.Net.AndroidMessageHandler.
• System.Net.Http.SocketsHttpHandler, System.Net.Http.

We should raise an error if the user is using an invalid value.

[jonathanpeppers]

The actual code changes look good. But maybe we should adjust some of the user-facing messages?

[jonpryor]

WIP commit message:

[Xamarin.Android.Build.Tasks] Add XA1031 error (#7448)

Fixes: https://github.com/xamarin/xamarin-android/issues/7326

Commit d3e87674 added a requirement that, under .NET 6+, the
`$(AndroidHttpClientHandlerType)` MUST inherit from
[`System.Net.Http.HttpMessageHandler`][0].  This was unfortunately a
change from Classic, which allowed/required that
[`System.Net.Http.HttpClientHandler`][1] be the base class.
Allowing `HttpClientHandler` to be in the inheritance hierarchy for
`$(AndroidHttpClientHandlerType)` would result in a
`NullReferenceException` at runtime under .NET 6.

Commit d3e87674 contained a TODO:

> TODO: [Emit a build-time warning][2] if
> `$(AndroidHttpClientHandlerType)` is set to an invalid value

Implement this TODO: add a build-time check that verifies that
`$(AndroidHttpClientHandlerType)` is a valid type for the target,
e.g. is an `HttpMessageHandler` subclass on .NET 6.

If `$(AndroidHttpClientHandlerType)` is not a valid type, then an
XA1031 error is raised:

	error XA1031: The 'AndroidHttpClientHandlerType' has an invalid value of '{0}' please check your project settings.

Additionally, if `$(AndroidHttpClientHandlerType)` is
`System.Net.Http.SocketsHttpHandler, System.Net.Http`, *and*
`$(UseNativeHttpHandler)` isn't set, then
set `$(UseNativeHttpHandler)` to false.  This is necessary to ensure
that `SocketsHttpHandler` is preserved by the linker.

[0]: https://learn.microsoft.com/en-us/dotnet/api/system.net.http.httpmessagehandler?view=net-6.0
[1]: https://learn.microsoft.com/en-us/dotnet/api/system.net.http.httpclienthandler?view=net-6.0
[2]: https://github.com/xamarin/xamarin-android/issues/7326

[jonpryor]

@dellis1972 : this feels like something that should be called out in the commit message. What's the rationale here?

[dellis1972]

I think this is left over. But we got info from the runtime team that if a user defines System.Net.Http.SocketsHttpHandler' as the handler type they MUST use set the UseNativeHttpHandler msbuild property. Otherwise it doesn't work.

This is where the conversaton took place https://discord.com/channels/732297728826277939/732297837953679412/1029777522897993748

[jonpryor]

Should we remove it ("this is left over") or keep it ("extract rationale from Discord")?