[Xamarin.Android.Build.Tasks] set %(DefineConstantsOnly) for older …

…API levels

Fixes: dotnet#8331
Context: dotnet#8569
Context: dotnet/sdk@25b360d

Building for `net9.0-android33` would give a poor error message:

    error NETSDK1181: Error getting pack version: Pack 'Microsoft.Android.Ref.33' was not present in workload manifests.
    C:\Program Files\dotnet\sdk\8.0.100-preview.7.23376.3\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.Sdk.FrameworkReferenceResolution.targets

To solve this, you would either change 33 to 34, or just remove the number to rely on the default value.

To make this easier:

* Automatically switch to 34 if the user specifies 33 or less.

* Opt into `%(DefineConstantsOnly)=true` for API levels 21-33 (and not
  the latest), which allows the .NET SDK to emit the *better* .NET SDK
  error message instead.

So now users will get:

    (_CheckForInvalidTargetPlatformVersion target) ->
    dotnet/sdk/9.0.100-alpha.1.23628.5/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.TargetFrameworkInference.targets(243,5):
    error NETSDK1140: 33.0 is not a valid TargetPlatformVersion for Android.
    Valid versions include: 34.0

I added a test for this scenario.

build-tools/Xamarin.Android.Tools.BootstrapTasks/Xamarin.Android.Tools.BootstrapTasks/GenerateSupportedPlatforms.cs

@@ -78,6 +78,9 @@ public override bool Execute ()
 						.OrderBy (v => v)) {
 					writer.WriteStartElement ("AndroidSdkSupportedTargetPlatformVersion");
 					writer.WriteAttributeString ("Include", apiLevel.ToString ("0.0", CultureInfo.InvariantCulture));
+					if (apiLevel < TargetApiLevel) {
+						writer.WriteAttributeString ("DefineConstantsOnly", "true");
+					}
 					writer.WriteEndElement (); // </AndroidSdkSupportedTargetPlatformVersion>
 				}
 				writer.WriteStartElement ("SdkSupportedTargetPlatformVersion");

src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/in/Microsoft.Android.Sdk.BundledVersions.in.targets

@@ -10,12 +10,16 @@ WARNING:  DO NOT MODIFY this file unless you are knowledgeable about MSBuild and
   <PropertyGroup>
     <AndroidNETSdkVersion>@ANDROID_PACK_VERSION_LONG@</AndroidNETSdkVersion>
     <XamarinAndroidVersion>@ANDROID_PACK_VERSION_LONG@</XamarinAndroidVersion>
+    <_AndroidLatestStableApiLevel>@ANDROID_LATEST_STABLE_API_LEVEL@</_AndroidLatestStableApiLevel>
   </PropertyGroup>
   <PropertyGroup>
     <_AndroidTargetingPackId Condition="$(TargetPlatformVersion.EndsWith('.0'))">$(TargetPlatformVersion.Substring(0, $(TargetPlatformVersion.LastIndexOf('.0'))))</_AndroidTargetingPackId>
     <_AndroidTargetingPackId Condition="'$(_AndroidTargetingPackId)' == ''">$(TargetPlatformVersion)</_AndroidTargetingPackId>
-    <_AndroidRuntimePackId Condition=" '$(_AndroidTargetingPackId)' &lt; '@ANDROID_LATEST_STABLE_API_LEVEL@' ">@ANDROID_LATEST_STABLE_API_LEVEL@</_AndroidRuntimePackId>
+    <!-- NOTE: adjust if a TargetFramework supports multiple API levels -->
+    <_AndroidErrorOnTargetPlatformVersion Condition=" '$(_AndroidTargetingPackId)' != '$(_AndroidLatestStableApiLevel)' ">$(_AndroidTargetingPackId)</_AndroidErrorOnTargetPlatformVersion>
+    <_AndroidTargetingPackId Condition=" '$(_AndroidTargetingPackId)' != '$(_AndroidLatestStableApiLevel)' ">$(_AndroidLatestStableApiLevel)</_AndroidTargetingPackId>
     <_AndroidRuntimePackId Condition=" '$(_AndroidRuntimePackId)' == '' ">$(_AndroidTargetingPackId)</_AndroidRuntimePackId>
+    <_AndroidRuntimePackId Condition=" '$(_AndroidRuntimePackId)' != '$(_AndroidLatestStableApiLevel)' ">$(_AndroidLatestStableApiLevel)</_AndroidRuntimePackId>
   </PropertyGroup>
   <ItemGroup>
     <KnownFrameworkReference

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

@@ -792,6 +792,20 @@ public void IfAndroidJarDoesNotExistThrowXA5207 ([Values(true, false)] bool buil
 			Directory.Delete (AndroidSdkDirectory, recursive: true);
 		}
 
+		[Test]
+		public void InvalidTargetPlatformVersion ([Values ("android33", "android99.0")] string platformVersion)
+		{
+			const string targetFramework = "net9.0";
+			var project = new XamarinAndroidApplicationProject {
+				TargetFramework = $"{targetFramework}-{platformVersion}",
+			};
+			using var builder = CreateApkBuilder ();
+			builder.ThrowOnBuildFailure = false;
+			Assert.IsFalse (builder.Build (project), "build should fail");
+
+			Assert.IsTrue (builder.LastBuildOutput.ContainsText ("error NETSDK1140:"), "NETSDK1140 should have been raised.");
+		}
+
 		[Test]
 		public void XA4212 ()
 		{