[Xamarin.Android.Build.Tasks] _AddMultiDexDependencyJars only runs for apps

Context: dotnet/android-libraries#247

When porting AndroidX to .NET 6, I noticed something odd... I was
getting `.aar` files in the build output that appeared to contain the
contents of `android-support-multidex.jar`.

Then I noticed:

https://github.com/xamarin/AndroidX/blob/16b02ae51980f7bab7be395d44f9998eea9bf32f/source/AndroidXProject.cshtml#L18

The AndroidX binding projects are setting `$(AndroidEnableMultiDex)`?

This is surely not intentional to be set in a class library, but this
causes `_AddMultiDexDependencyJars` to run:

    <Target Name="_AddMultiDexDependencyJars">
      <!-- ... -->
      <ItemGroup Condition=" '$(AndroidEnableMultiDex)' == 'True' AND '$(AndroidMultiDexSupportJar)' == '' ">
        <AndroidJavaLibrary Include="$(MonoAndroidToolsDirectory)\android-support-multidex.jar" />
      </ItemGroup>
    </Target>

If this was added to `@(AndroidJavaLibrary)`, then the `<CreateAar/>`
MSBuild task would happily package it in a .NET 6 class library!

I think the solution here is that the `_AddMultiDexDependencyJars`
MSBuild target shouldn't actually run unless `$(AndroidApplication)`
is `true`.

I added updated an existing test for this scenario.

Author: jonathanpeppers

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

@@ -175,9 +175,30 @@ public void ClassLibraryHasNoWarnings ()
 			//NOTE: these properties should not affect class libraries at all
 			proj.SetProperty ("AndroidPackageFormat", "aab");
 			proj.SetProperty ("AotAssemblies", "true");
+			proj.SetProperty ("AndroidEnableMultiDex", "true");
 			using (var b = CreateDllBuilder ()) {
 				Assert.IsTrue (b.Build (proj), "Build should have succeeded.");
 				Assert.IsTrue (StringAssertEx.ContainsText (b.LastBuildOutput, " 0 Warning(s)"), "Should have no MSBuild warnings.");
+
+				// $(AndroidEnableMultiDex) should not add android-support-multidex.jar!
+				if (Builder.UseDotNet) {
+					var aarPath = Path.Combine (Root, b.ProjectDirectory, proj.OutputPath, $"{proj.ProjectName}.aar");
+					using var zip = Xamarin.Tools.Zip.ZipArchive.Open (aarPath, FileMode.Open);
+					Assert.IsFalse (zip.Any (e => e.FullName.EndsWith (".jar", StringComparison.OrdinalIgnoreCase)),
+						$"{aarPath} should not contain a .jar file!");
+				} else {
+					var assemblyPath = Path.Combine (Root, b.ProjectDirectory, proj.OutputPath, $"{proj.ProjectName}.dll");
+					using var assembly = AssemblyDefinition.ReadAssembly (assemblyPath);
+					const string libraryProjects = "__AndroidLibraryProjects__.zip";
+					var resource = assembly.MainModule.Resources.OfType<EmbeddedResource> ()
+						.FirstOrDefault (e => e.Name == libraryProjects);
+					Assert.IsNotNull (resource, $"{assemblyPath} should contain {libraryProjects}");
+
+					using var stream = resource.GetResourceStream ();
+					using var zip = Xamarin.Tools.Zip.ZipArchive.Open (stream);
+					Assert.IsFalse (zip.Any (e => e.FullName.EndsWith (".jar", StringComparison.OrdinalIgnoreCase)),
+						$"{resource.Name} should not contain a .jar file!");
+				}
 			}
 		}
 

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

@@ -1021,7 +1021,8 @@ because xbuild doesn't support framework reference assemblies.
 	</ItemGroup>
 </Target>
 
-<Target Name="_AddMultiDexDependencyJars">
+<Target Name="_AddMultiDexDependencyJars"
+    Condition=" '$(AndroidApplication)' == 'true' ">
   <ItemGroup Condition=" '$(AndroidEnableMultiDex)' == 'True' AND '$(AndroidMultiDexSupportJar)' != '' ">
     <AndroidJavaLibrary Include="$(_AndroidSdkDirectory)\$(AndroidMultiDexSupportJar)" />
   </ItemGroup>