[tests, build] multi-target Xamarin.Android.Build.Tests

[jonathanpeppers]

There are various tests in Xamarin.Android.Build.Tests that execute
MSBuild tasks directly. This means the changes in #4820 are not quite
complete. We need to actually run the tests under .NET Core to see
if these tests would pass under a One .NET context.

The solution (you would think), would be to make the test assembly
netstandard2.0 so you could run either:

> nunit3-console Xamarin.Android.Build.Tests.dll
> dotnet test Xamarin.Android.Build.Tests.dll

nunit3-console would execute the tests under .NET framework/Mono and
dotnet test would run under .NET Core or .NET 5+.

Unfortunately, nunit3-console can't run tests from a
netstandard2.0 library!

So we multi-target:

<TargetFrameworks>net472;net5.0</TargetFrameworks>

Unfortunately, the net5.0 causes MSBuild to fail with:

> msbuild Xamarin.Android.sln
...
(GetReferenceAssemblyPaths target) ->
  C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\MSBuild\Current\Bin\Microsoft.Common.CurrentVersion.targets(1177,5): error MSB3644: The reference assemblies for .NETFramework,Version=v5.0 were not found. To resolve this, install the Developer Pack (SDK/Targeting Pack) for this framework version or retarget your application. You can download .NET Framework Developer Packs at https://aka.ms/msbuild/developerpacks

I think there is some expectation to use dotnet build for .NET 5
instead. We don't want to move our entire build over to dotnet build yet (we probably can't!), so instead we can do:

<TargetFrameworks>net472;netcoreapp3.1</TargetFrameworks>

This allows msbuild Xamarin.Android.sln to continue to work. This
might also be something that gets fixed in a future MSBuild version.

I updated various .yaml definitions so we can run NUnit tests via
nunit3-console or dotnet test as needed.

Other things that broke here:

• We need Microsoft.NET.Test.Sdk for dotnet test to work.
• Builder.UseDotNet can now be set based on a #if instead of an
  NUnit parameter.
• Xamarin.Android.Tools.Aidl needs to be netstandard2.0. This
  required including a System.CodeDom package.
• Xamarin.Android.Build.Tasks can't use FSharp.Core in
  netcoreapp3.1. I just removed it, since it is not used.
• Updated various paths to Xamarin.Android.Build.Tests.dll.

[jonathanpeppers]

Here is a build with a full run of the One .NET tests: https://build.azdo.io/3979182

[jonathanpeppers]

I checked src\Xamarin.Android.Build.Tasks\obj\Debug\Profile.g.cs and FSharp.Core.dll isn't being written to it at all, after this was removed:

using System.Collections.Generic;

namespace Xamarin.Android.Tasks {
	public partial class Profile {
		// KEEP THIS SORTED ALPHABETICALLY, CASE-INSENSITIVE
		public static readonly string [] SharedRuntimeAssemblies = new []{
			"I18N.CJK.dll",
			"I18N.dll",
			"I18N.MidEast.dll",
			"I18N.Other.dll",
			"I18N.Rare.dll",
			"I18N.West.dll",
			"Java.Interop.dll",
			"Microsoft.CSharp.dll",
			"Mono.Android.dll",
			"Mono.Android.Export.dll",
			"Mono.Btls.Interface.dll",
			"Mono.CompilerServices.SymbolWriter.dll",
			"Mono.CSharp.dll",
			"Mono.Data.Sqlite.dll",
			"Mono.Data.Tds.dll",
			"Mono.Posix.dll",
			"Mono.Security.dll",
			"mscorlib.dll",
			"OpenTK.dll",
			"OpenTK-1.0.dll",
			"System.ComponentModel.Composition.dll",
			"System.ComponentModel.DataAnnotations.dll",
			"System.Core.dll",
			"System.Data.DataSetExtensions.dll",
			"System.Data.dll",
			"System.Data.Services.Client.dll",
			"System.dll",
			"System.EnterpriseServices.dll",
			"System.IdentityModel.dll",
			"System.IO.Compression.dll",
			"System.IO.Compression.FileSystem.dll",
			"System.Json.dll",
			"System.Net.dll",
			"System.Net.Http.dll",
			"System.Net.Http.WinHttpHandler.dll",
			"System.Numerics.dll",
			"System.Numerics.Vectors.dll",
			"System.Reflection.Context.dll",
			"System.Runtime.CompilerServices.Unsafe.dll",
			"System.Runtime.Serialization.dll",
			"System.Security.dll",
			"System.ServiceModel.dll",
			"System.ServiceModel.Internals.dll",
			"System.ServiceModel.Web.dll",
			"System.Transactions.dll",
			"System.Web.Services.dll",
			"System.Windows.dll",
			"System.Xml.dll",
			"System.Xml.Linq.dll",
			"System.Xml.Serialization.dll",
		};
	}
}