[One .NET] conditionally add generated\*\*.cs to @(Compile) (#6291)

Fixes: #6280

.NET 6 projects have the following issue:

 1. Build a .NET 6 project with a `.jar` to create a C# binding.
 2. Delete the `.jar` completely & build again.
 3. The generated C# binding types remain in the assembly!

We have the following expression to decide when C#-binding related
targets should be completely skipped:

	Condition=" '$(UsingAndroidNETSdk)' != 'true' Or '@(InputJar->Count())' != '0' Or '@(EmbeddedJar->Count())' != '0' Or '@(LibraryProjectZip->Count())' != '0' "

This allows the targets to run for "legacy" Xamarin.Android binding
projects, and .NET 6 project when certain item groups are non-empty.

We were missing this condition on the `AddBindingsToCompile` MSBuild
target!

	<Target Name="AddBindingsToCompile" DependsOnTargets="GenerateBindings">
	  <ItemGroup>
	    <Compile Include="$(IntermediateOutputPath)generated\*\*.cs" />
	  </ItemGroup>
	</Target>

I don't think we necessarily have to delete this directory; these
files should just not be added to the `@(Compile)` item group.

Instead of copying this long `Condition` to a third place, refactor
the expression into a new `_SetAndroidGenerateManagedBindings`
MSBuild target.  All three places can use a simpler expression that
will be harder to mess up:

	Condition=" '$(_AndroidGenerateManagedBindings)' == 'true' "

I updated a test to verify removing a `.jar` removes the C# classes
from the compiled assembly.

src/Xamarin.Android.Build.Tasks/MSBuild/Xamarin/Android/Xamarin.Android.Bindings.Core.targets

@@ -28,16 +28,24 @@ It is shared between "legacy" binding projects and .NET 5 projects.
     <NoWarn Condition=" '$(DocumentationFile)' != '' ">$(NoWarn);CS1573;CS1591</NoWarn>
   </PropertyGroup>
 
-  <Target Name="ExportJarToXml"
-      DependsOnTargets="$(ExportJarToXmlDependsOnTargets)"
+  <Target Name="_SetAndroidGenerateManagedBindings"
       Condition=" '$(UsingAndroidNETSdk)' != 'true' Or '@(InputJar->Count())' != '0' Or '@(EmbeddedJar->Count())' != '0' Or '@(LibraryProjectZip->Count())' != '0' ">
+    <PropertyGroup>
+      <!-- Used throughout to determine if C# binding-related targets should skip -->
+      <_AndroidGenerateManagedBindings>true</_AndroidGenerateManagedBindings>
+    </PropertyGroup>
+  </Target>
+
+  <Target Name="ExportJarToXml"
+      Condition=" '$(_AndroidGenerateManagedBindings)' == 'true' "
+      DependsOnTargets="$(ExportJarToXmlDependsOnTargets)">
     <PropertyGroup>
       <AllowUnsafeBlocks Condition=" '$(AllowUnsafeBlocks)' != 'true' ">true</AllowUnsafeBlocks>
     </PropertyGroup>
   </Target>
 
   <Target Name="GenerateBindings"
-      Condition=" '$(UsingAndroidNETSdk)' != 'true' Or '@(InputJar->Count())' != '0' Or '@(EmbeddedJar->Count())' != '0' Or '@(LibraryProjectZip->Count())' != '0' "
+      Condition=" '$(_AndroidGenerateManagedBindings)' == 'true' "
       DependsOnTargets="ExportJarToXml;_ResolveMonoAndroidSdks"
       Inputs="$(ApiOutputFile);@(TransformFile);@(ReferencePath);@(ReferenceDependencyPaths);@(_AndroidMSBuildAllProjects)"
       Outputs="$(_GeneratorStampFile)">
@@ -79,7 +87,9 @@ It is shared between "legacy" binding projects and .NET 5 projects.
 
   </Target>
 
-  <Target Name="AddBindingsToCompile" DependsOnTargets="GenerateBindings">
+  <Target Name="AddBindingsToCompile"
+      Condition=" '$(_AndroidGenerateManagedBindings)' == 'true' "
+      DependsOnTargets="GenerateBindings">
     <!-- Add the files to list of things to be compiled -->
     <ItemGroup>
       <Compile Include="$(IntermediateOutputPath)generated\*\*.cs" />

src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.BuildOrder.targets

@@ -112,6 +112,7 @@ projects, these properties are set in Xamarin.Android.Legacy.targets.
       $(CoreResolveReferencesDependsOn);
       UpdateAndroidInterfaceProxies;
       UpdateAndroidResources;
+      _SetAndroidGenerateManagedBindings;
       AddBindingsToCompile;
     </ResolveReferencesDependsOn>
     <_UpdateAndroidResourcesDependsOn>

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/XASdkTests.cs

@@ -300,14 +300,24 @@ public void AppWithSingleJar ()
 			};
 
 			var dotnet = CreateDotNetBuilder (proj);
-			Assert.IsTrue (dotnet.Build (), "build should succeed");
+			Assert.IsTrue (dotnet.Build (), "first build should succeed");
 
 			var assemblyPath = Path.Combine (FullProjectDirectory, proj.OutputPath, $"{proj.ProjectName}.dll");
+			var typeName = "Com.Xamarin.Android.Test.Msbuildtest.JavaSourceJarTest";
 			FileAssert.Exists (assemblyPath);
 			using (var assembly = AssemblyDefinition.ReadAssembly (assemblyPath)) {
-				var typeName = "Com.Xamarin.Android.Test.Msbuildtest.JavaSourceJarTest";
 				Assert.IsNotNull (assembly.MainModule.GetType (typeName), $"{assemblyPath} should contain {typeName}");
 			}
+
+			// Remove the @(AndroidLibrary) & build again
+			proj.Sources.RemoveAt (proj.Sources.Count - 1);
+			Directory.Delete (Path.Combine (FullProjectDirectory, "Jars"), recursive: true);
+			Assert.IsTrue (dotnet.Build (), "second build should succeed");
+
+			FileAssert.Exists (assemblyPath);
+			using (var assembly = AssemblyDefinition.ReadAssembly (assemblyPath)) {
+				Assert.IsNull (assembly.MainModule.GetType (typeName), $"{assemblyPath} should *not* contain {typeName}");
+			}
 		}
 
 		[Test]

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Legacy.targets

@@ -187,6 +187,7 @@ projects. .NET 5 projects will not import this file.
 
     <ResolveReferencesDependsOn>
       $(ResolveReferencesDependsOn);
+      _SetAndroidGenerateManagedBindings;
       AddBindingsToCompile;
       AddEmbeddedJarsAsResources;
       AddEmbeddedReferenceJarsAsResources;