[Xamarin.Android.Build.Tasks] require JDK 11 for API 31

API 31 builds fail when using JDK 1.8 with:

    /Library/Frameworks/Mono.framework/External/xbuild/Xamarin/Android/Xamarin.Android.Common.targets(1728,3):
    error JAVAC0000: java.lang.AssertionError: annotationType() : unrecognized Attribute name MODULE (class com.sun.tools.javac.util.SharedNameTable$NameImpl)

To solve this issue, we must require JDK 11 for API 31.

There are two code paths here for "legacy" Xamarin.Android and .NET 6.
Legacy is straightforward, as we already had a pattern for this.

In .NET 6, I checked the `$(TargetPlatformVersion)` property for now
and require JDK 11 in that case. When API 31 is stable, we can
probably remove this code and just set this value by default:

    <MinimumSupportedJavaVersion Condition=" '$(MinimumSupportedJavaVersion)' == '' ">11.0</MinimumSupportedJavaVersion>

I updated some tests in this area as well.

Author: jonathanpeppers

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

@@ -23,6 +23,7 @@ called for "legacy" projects in Xamarin.Android.Legacy.targets.
         JavaSdkPath="$(_JavaSdkDirectory)"
         JavaToolExe="$(JavaToolExe)"
         JavacToolExe="$(JavacToolExe)"
+        TargetPlatformVersion="$(TargetPlatformVersion)"
         LatestSupportedJavaVersion="$(LatestSupportedJavaVersion)"
         MinimumSupportedJavaVersion="$(MinimumSupportedJavaVersion)">
       <Output TaskParameter="JdkVersion"                PropertyName="_JdkVersion" />

src/Xamarin.Android.Build.Tasks/Tasks/Legacy/ValidateJavaVersion.cs

@@ -50,13 +50,8 @@ protected override bool ValidateJava (string javaExe, Regex versionRegex)
 		Version GetJavaVersionForFramework ()
 		{
 			var apiLevel = MonoAndroidHelper.SupportedVersions.GetApiLevelFromFrameworkVersion (TargetFrameworkVersion);
-			if (apiLevel >= 30) {
-				// At present, it *looks like* API-R works with Build-tools r29, but
-				// historically API-X requires Build-tools rX, so if/when API-30
-				// requires Build-tools r30, it will require JDK11.
-				// return new Version (11, 0);
-				return new Version (1, 8);
-			}
+			if (apiLevel >= 31)
+				return new Version (11, 0);
 			if (apiLevel >= 24)
 				return new Version (1, 8);
 			else if (apiLevel == 23)

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

@@ -21,6 +21,8 @@ public class ValidateJavaVersion : AndroidTask
 
 		public string JavacToolExe { get; set; }
 
+		public string TargetPlatformVersion { get; set; }
+
 		[Required]
 		public string LatestSupportedJavaVersion { get; set; }
 
@@ -67,6 +69,11 @@ bool ValidateJava ()
 		protected virtual bool ValidateJava (string javaExe, Regex versionRegex)
 		{
 			var required = Version.Parse (MinimumSupportedJavaVersion);
+			// NOTE: when API 31 is stable and the default, we should
+			// remove this check and rely solely on $(MinimumSupportedJavaVersion).
+			if (Version.TryParse (TargetPlatformVersion, out var targetPlatformVersion) && targetPlatformVersion.Major >= 31) {
+				required = new Version (11, 0);
+			}
 			MinimumRequiredJdkVersion = required.ToString ();
 
 			try {

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

@@ -21,7 +21,10 @@ public class ResolveSdksTaskTests : BaseTest {
 			new ApiInfo () { Id = "25", Level = 25, Name = "Nougat", FrameworkVersion = "v7.1",  Stable = true },
 			new ApiInfo () { Id = "26", Level = 26, Name = "Oreo", FrameworkVersion = "v8.0",  Stable = true  },
 			new ApiInfo () { Id = "27", Level = 27, Name = "Oreo", FrameworkVersion = "v8.1",  Stable = true  },
-			new ApiInfo () { Id = "P",  Level = 28, Name = "P",    FrameworkVersion = "v8.99", Stable = false },
+			new ApiInfo () { Id = "28", Level = 28, Name = "Pie",    FrameworkVersion = "v9.0", Stable = false },
+			new ApiInfo () { Id = "29", Level = 29, Name = "Android10", FrameworkVersion = "v10.0", Stable = false },
+			new ApiInfo () { Id = "30", Level = 30, Name = "Android11", FrameworkVersion = "v11.0", Stable = false },
+			new ApiInfo () { Id = "S",  Level = 31, Name = "S",         FrameworkVersion = "v11.0.99", Stable = false },
 			new ApiInfo () { Id = "Z",  Level = 127, Name = "Z",    FrameworkVersion = "v108.1.99", Stable = false },
 		};
 
@@ -35,9 +38,9 @@ public class ResolveSdksTaskTests : BaseTest {
 				/* jdk */ "1.8.0",
 				/* apis*/ apiInfoSelection,
 				/* useLatestAndroidSdk */ true,
-				/* targetFrameworkVersion */ "v8.99",
+				/* targetFrameworkVersion */ "v9.0",
 				/* expectedTaskResult */ true,
-				/* expectedTargetFramework */ "v8.99",
+				/* expectedTargetFramework */ "v9.0",
 				/* expectedError */ "",
 				/* expectedErrorMessage */ "",
 			},
@@ -101,9 +104,9 @@ public class ResolveSdksTaskTests : BaseTest {
 				/* jdk */ "1.8.0",
 				/* apis*/ apiInfoSelection,
 				/* useLatestAndroidSdk */ false,
-				/* targetFrameworkVersion */ "v8.99",
+				/* targetFrameworkVersion */ "v9.0",
 				/* expectedTaskResult */ true,
-				/* expectedTargetFramework */ "v8.99",
+				/* expectedTargetFramework */ "v9.0",
 				/* expectedError */ "",
 				/* expectedErrorMessage */ "",
 			},
@@ -151,6 +154,17 @@ public class ResolveSdksTaskTests : BaseTest {
 				/* expectedError */ "XA0001",
 				/* expectedErrorMessage */ "Unsupported or invalid $(TargetFrameworkVersion) value of 'v6.0'. Please update your Project Options.",
 			},
+			new object[] {
+				/* buildtools */   "30.0.0",
+				/* jdk */ "11.0",
+				/* apis*/ apiInfoSelection,
+				/* useLatestAndroidSdk */ false,
+				/* targetFrameworkVersion */ "v11.0.99",
+				/* expectedTaskResult */ true,
+				/* expectedTargetFramework */ "v11.0.99",
+				/* expectedError */ "",
+				/* expectedErrorMessage */ "",
+			},
 		};
 		#pragma warning restore 414
 		[Test]
@@ -184,7 +198,7 @@ public void UseLatestAndroidSdk (string buildtools, string jdk, ApiInfo[] apis,
 				JavaSdkPath = javaPath,
 				JavaToolExe = javaExe,
 				JavacToolExe = javacExe,
-				LatestSupportedJavaVersion = "1.8.0",
+				LatestSupportedJavaVersion = LatestSupportedJavaVersion,
 				MinimumSupportedJavaVersion = "1.7.0",
 			};
 			var androidTooling = new ResolveAndroidTooling {