[One .NET] prefer /manifest/@android:versionCode (#6383)

Fixes: dotnet/sdk#21614
Fixes: #6384

Context: 84d0d9b
Context: https://github.com/xamarin/xamarin-android/blob/650f289301583003aa4acb075923bcc5627b92dc/Documentation/guides/OneDotNetSingleProject.md

In .NET 6, if [`/manifest/@android:versionCode`][0] is within
[`AndroidManifest.xml`][1], it won't be used, as it is always
overwritten by a default value:

	<ApplicationVersion Condition=" '$(ApplicationVersion)' == '' ">1</ApplicationVersion>

The other new MSBuild properties don't appear to suffer from this
problem; values within `Properties\AndroidManifest.xml` always take
precedence over MSBuild properties, [as documented in the spec][2]:

  * `$(ApplicationDisplayVersion)`
  * `$(ApplicationId)`
  * `$(ApplicationTitle)`

This is because we don't default these properties to a value when
they are blank.

Fix this scenario by updating the `<GenerateJavaStubs/>` task to only
set `ManifestDocument.VersionCode` when
`ManifestDocument.HasVersionCode` is false and the version code is set.

I added a new test for this scenario.

[0]: https://developer.android.com/guide/topics/manifest/manifest-element#vcode
[1]: https://developer.android.com/guide/topics/manifest/manifest-intro
[2]: https://github.com/xamarin/xamarin-android/blob/650f289301583003aa4acb075923bcc5627b92dc/Documentation/guides/OneDotNetSingleProject.md

src/Xamarin.Android.Build.Tasks/Tasks/GenerateJavaStubs.cs

@@ -282,9 +282,10 @@ void Run (DirectoryAssemblyResolver res)
 				NeedsInternet = NeedsInternet,
 				InstantRunEnabled = InstantRunEnabled
 			};
-			// Only set manifest.VersionCode if it is not blank.
-			// We do not want to override the existing manifest in this case.
-			if (!string.IsNullOrEmpty (VersionCode)) {
+			// Only set manifest.VersionCode if there is no existing value in AndroidManifest.xml.
+			if (manifest.HasVersionCode) {
+				Log.LogDebugMessage ($"Using existing versionCode in: {ManifestTemplate}");
+			} else if (!string.IsNullOrEmpty (VersionCode)) {
 				manifest.VersionCode = VersionCode;
 			}
 			manifest.Assemblies.AddRange (userAssemblies.Values);

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

@@ -108,5 +108,39 @@ public void AndroidManifestProperties (string versionName, string versionCode, s
 				Assert.AreEqual (versionNumber, assemblyFileVersion.ConstructorArguments [0].Value);
 			}
 		}
+
+		[Test]
+		public void AndroidManifestValuesWin ()
+		{
+			var packageName = "com.xamarin.singleproject";
+			var applicationLabel = "My Sweet App";
+			var versionName = "99.0";
+			var versionCode = "99";
+			var proj = new XamarinAndroidApplicationProject ();
+			proj.AndroidManifest = proj.AndroidManifest
+				.Replace ("package=\"${PACKAGENAME}\"", $"package=\"{packageName}\"")
+				.Replace ("android:label=\"${PROJECT_NAME}\"", $"android:label=\"{applicationLabel}\"")
+				.Replace ("android:versionName=\"1.0\"", $"android:versionName=\"{versionName}\"")
+				.Replace ("android:versionCode=\"1\"", $"android:versionCode=\"{versionCode}\"");
+			if (!Builder.UseDotNet) {
+				proj.SetProperty ("GenerateApplicationManifest", "true");
+			}
+			proj.SetProperty ("ApplicationId", "com.i.should.not.be.used");
+			proj.SetProperty ("ApplicationTitle", "I should not be used");
+			proj.SetProperty ("ApplicationVersion", "21");
+			proj.SetProperty ("ApplicationDisplayVersion", "1.1.1.1");
+
+			using var b = CreateApkBuilder ();
+			Assert.IsTrue (b.Build (proj), "Build should have succeeded.");
+			var manifest = b.Output.GetIntermediaryPath ("android/AndroidManifest.xml");
+			FileAssert.Exists (manifest);
+
+			using var stream = File.OpenRead (manifest);
+			var doc = XDocument.Load (stream);
+			Assert.AreEqual (packageName, doc.Root.Attribute ("package")?.Value);
+			Assert.AreEqual (versionName, doc.Root.Attribute (AndroidAppManifest.AndroidXNamespace + "versionName")?.Value);
+			Assert.AreEqual (versionCode, doc.Root.Attribute (AndroidAppManifest.AndroidXNamespace + "versionCode")?.Value);
+			Assert.AreEqual (applicationLabel, doc.Root.Element ("application").Attribute (AndroidAppManifest.AndroidXNamespace + "label")?.Value);
+		}
 	}
 }

src/Xamarin.Android.Build.Tasks/Utilities/ManifestDocument.cs

@@ -36,6 +36,7 @@ internal class ManifestDocument
 
 		static XNamespace androidNs = AndroidXmlNamespace;
 		static XNamespace androidToolsNs = AndroidXmlToolsNamespace;
+		static readonly XName versionCodeAttributeName = androidNs + "versionCode";
 
 		XDocument doc;
 
@@ -101,7 +102,7 @@ internal class ManifestDocument
 		/// </summary>
 		public string VersionCode {
 			get {
-				XAttribute attr = doc.Root.Attribute (androidNs + "versionCode");
+				XAttribute attr = doc.Root.Attribute (versionCodeAttributeName);
 				if (attr != null) {
 					string code = attr.Value;
 					if (!string.IsNullOrEmpty (code))
@@ -110,10 +111,12 @@ internal class ManifestDocument
 				return "1";
 			}
 			set {
-				doc.Root.SetAttributeValue (androidNs + "versionCode", versionCode = value);
+				doc.Root.SetAttributeValue (versionCodeAttributeName, versionCode = value);
 			}
 		}
 
+		public bool HasVersionCode => doc.Root.Attribute (versionCodeAttributeName) != null;
+
 		public string GetMinimumSdk () {
 			int defaultMinSdkVersion = MonoAndroidHelper.SupportedVersions.MinStableVersion.ApiLevel;
 			var minAttr = doc.Root.Element ("uses-sdk")?.Attribute (androidNs + "minSdkVersion");
@@ -270,8 +273,8 @@ public IList<string> Merge (TaskLoggingHelper log, TypeDefinitionCache cache, Li
 
 			manifest.SetAttributeValue (XNamespace.Xmlns + "android", "http://schemas.android.com/apk/res/android");
 
-			if (manifest.Attribute (androidNs + "versionCode") == null) {
-				manifest.SetAttributeValue (androidNs + "versionCode",
+			if (manifest.Attribute (versionCodeAttributeName) == null) {
+				manifest.SetAttributeValue (versionCodeAttributeName,
 					string.IsNullOrEmpty (versionCode) ? "1" : versionCode);
 			}
 			if (manifest.Attribute (androidNs + "versionName") == null) {