[Xamarin.Android.Build.Tasks] Avoid XA5207 for design-time builds (#7434)

Fixes: #7405

Raising an XA5207 error during a [design time build][0] in
Visual Studio (Windows) can cause problems in the auto sdk installer:

	error XA5207: Could not find android.jar for API level 31.
	  This means the Android SDK platform for API level 31 is not installed.
	  Either install it in the Android SDK Manager (Tools > Android > Android SDK Manager…),
	  or change the Xamarin.Android project to target an API version that is installed.
	  (C:\Program Files (x86)\Android\android-sdk\platforms\android-31\android.jar missing.)

The design time build is run for all frameworks; you can end up in a
position where you have the Windows framework selected in the IDE and
the auto sdk installer will try to install an Android framework.
Its very confusing for the user.

Avoid raising error XA5207 during design time builds.

[0]: https://github.com/dotnet/project-system/blob/3f5fb23d3fa91719c93b68f56e99aba6b18939aa/docs/design-time-builds.md

Author: dellis1972

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

@@ -23,6 +23,8 @@ public class GetJavaPlatformJar : AndroidTask
 
 		public string AndroidManifest { get; set; }
 
+		public bool DesignTimeBuild { get; set; }
+
 		[Output]
 		public string JavaPlatformJarPath { get; set; }
 
@@ -84,12 +86,11 @@ public override bool RunTask ()
 			}
 
 			platform            = GetTargetSdkVersion (platform, target_sdk);
-			JavaPlatformJarPath =  MonoAndroidHelper.TryGetAndroidJarPath (Log, platform);
+			JavaPlatformJarPath =  MonoAndroidHelper.TryGetAndroidJarPath (Log, platform, designTimeBuild: DesignTimeBuild);
+			TargetSdkVersion = MonoAndroidHelper.SupportedVersions.GetApiLevelFromId (platform).ToString ();
 			if (JavaPlatformJarPath == null)
 				return !Log.HasLoggedErrors;
 
-			TargetSdkVersion = MonoAndroidHelper.SupportedVersions.GetApiLevelFromId (platform).ToString ();
-
 			return !Log.HasLoggedErrors;
 		}
 

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

@@ -1365,6 +1365,8 @@ public void IfAndroidJarDoesNotExistThrowXA5207 ()
 				if (!Builder.UseDotNet)
 					proj.TargetFrameworkVersion = builder.LatestTargetFrameworkVersion ();
 				builder.ThrowOnBuildFailure = false;
+				Assert.IsTrue (builder.DesignTimeBuild (proj), "DesignTime build should succeed.");
+				Assert.IsFalse (builder.LastBuildOutput.ContainsText ("error XA5207:"), "XA5207 should not have been raised.");
 				builder.Target = "AndroidPrepareForBuild";
 				Assert.IsFalse (builder.Build (proj, parameters: new string [] {
 					$"AndroidSdkBuildToolsVersion=24.0.1",

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

@@ -443,13 +443,15 @@ public static IEnumerable<string> Executables (string executable)
 			yield return executable;
 		}
 
-		public static string TryGetAndroidJarPath (TaskLoggingHelper log, string platform)
+		public static string TryGetAndroidJarPath (TaskLoggingHelper log, string platform, bool designTimeBuild = false)
 		{
 			var platformPath = MonoAndroidHelper.AndroidSdk.TryGetPlatformDirectoryFromApiLevel (platform, MonoAndroidHelper.SupportedVersions);
 			if (platformPath == null) {
-				var expectedPath = MonoAndroidHelper.AndroidSdk.GetPlatformDirectoryFromId (platform);
-				var sdkManagerMenuPath = OS.IsWindows ? Properties.Resources.XA5207_SDK_Manager_Windows : Properties.Resources.XA5207_SDK_Manager_macOS;
-				log.LogCodedError ("XA5207", Properties.Resources.XA5207, platform, Path.Combine (expectedPath, "android.jar"), sdkManagerMenuPath);
+				if (!designTimeBuild) {
+					var expectedPath = MonoAndroidHelper.AndroidSdk.GetPlatformDirectoryFromId (platform);
+					var sdkManagerMenuPath = OS.IsWindows ? Properties.Resources.XA5207_SDK_Manager_Windows : Properties.Resources.XA5207_SDK_Manager_macOS;
+					log.LogCodedError ("XA5207", Properties.Resources.XA5207, platform, Path.Combine (expectedPath, "android.jar"), sdkManagerMenuPath);
+				}
 				return null;
 			}
 			return Path.Combine (platformPath, "android.jar");

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

@@ -99,7 +99,9 @@ projects.
   <Target Name="_GetJavaPlatformJar">
     <GetJavaPlatformJar
         AndroidSdkPlatform="$(_AndroidApiLevel)"
-        AndroidManifest="$(_AndroidManifestAbs)">
+        AndroidManifest="$(_AndroidManifestAbs)"
+        DesignTimeBuild="$(DesignTimeBuild)"
+    >
       <Output TaskParameter="JavaPlatformJarPath" PropertyName="JavaPlatformJarPath" />
       <Output TaskParameter="TargetSdkVersion"    PropertyName="_AndroidTargetSdkVersion" />
     </GetJavaPlatformJar>