[Tests] Bump default AGP to 9.1.0 for compileSdk 36 compatibility (#11043)

Fixes: #11042

## Summary

Bumps the default Android Gradle Plugin (AGP) version in the test infrastructure from **8.5.0** to **9.1.0** to fix Gradle test failures caused by an AGP/compileSdk version mismatch.

## Problem

The default `AgpVersion` was hardcoded to `8.5.0` in `AndroidGradleProject.cs`, while `CompileSdk` is dynamically derived from `XABuildConfig.AndroidDefaultTargetDotnetApiLevel` (now **36**). AGP 8.5.0 only supports up to compileSdk 34, causing a Gradle configuration-cache serialization crash in `MergeJavaResourceTask` for tests like `BuildMultipleModules(MonoVM)`.

See the [AGP compatibility table](https://d.android.com/r/tools/api-level-support) for details.

## Changes

- **`AndroidGradleProject.cs`**: Bump default `AgpVersion` from `"8.5.0"` → `"9.1.0"` and set default `GradleVersion` to `"9.1.0"`
- **`AndroidGradleProject.cs`**: Add optional `compileSdk` parameter to `CreateDefault` overload
- **`AndroidGradleProjectTests.cs`**: Fix `BindLibraryWithMultipleGradleVersions` — the AGP 8.5.0 test case now uses `compileSdk = 34` (within its supported range), while the AGP 9.0.0 case uses the current default

## Bump default Gradle version to 9.3.1 (minimum for AGP 9.1.0)

AGP 9.1.0 requires Gradle >= 9.3.1, not 9.1.0. The previous default
caused all Gradle-based tests to fail with:
'Minimum supported Gradle version is 9.3.1. Current version is 9.1.0.'

## Add android:exported="true" to test activity with intent filter

AGP 9.1.0 with compileSdk 36 enforces the Android 12 requirement that
activities with intent filters must explicitly declare android:exported.
The SetupDefaultApp() manifest template was missing this attribute,
causing BuildApp and BuildMultipleModules tests to fail with:
  android:exported needs to be explicitly specified for element
  <activity#com.example.TestModule.TestActivity>

Co-authored-by: Jonathan Peppers <jonathan.peppers@microsoft.com>

Author: simonrozsival

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

@@ -582,10 +582,11 @@ public void TestFacebook () {{
 		static IEnumerable<object[]> GetAgpGradleVersionTestData ()
 		{
 			// AGP 8.5.0 with Gradle 8.7 - baseline, tests existing behavior
-			yield return new object[] { "8.5.0", "8.7" };
+			// AGP 8.5.0 only supports up to compileSdk 34
+			yield return new object[] { "8.5.0", "8.7", 34 };
 			// AGP 9.0.0 with Gradle 9.1.0 - tests the Gradle 9.x stricter Kotlin type checking fix
 			// Note: Full version "9.1.0" is required for the download URL to work correctly
-			yield return new object[] { "9.0.0", "9.1.0" };
+			yield return new object[] { "9.0.0", "9.1.0", XABuildConfig.AndroidDefaultTargetDotnetApiLevel.Major };
 		}
 
 		/// <summary>
@@ -595,9 +596,9 @@ static IEnumerable<object[]> GetAgpGradleVersionTestData ()
 		/// </summary>
 		[Test]
 		[TestCaseSource (nameof (GetAgpGradleVersionTestData))]
-		public void BindLibraryWithMultipleGradleVersions (string agpVersion, string gradleVersion)
+		public void BindLibraryWithMultipleGradleVersions (string agpVersion, string gradleVersion, int compileSdk)
 		{
-			var gradleProject = AndroidGradleProject.CreateDefault (GradleTestProjectDir, agpVersion, gradleVersion);
+			var gradleProject = AndroidGradleProject.CreateDefault (GradleTestProjectDir, agpVersion, gradleVersion, compileSdk: compileSdk);
 			var gradleModule = gradleProject.Modules.First ();
 			var moduleName = gradleModule.Name;
 

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Android/AndroidGradleModule.cs

@@ -115,7 +115,7 @@ protected void onCreate(Bundle savedInstanceState) {{
 			});
 			AndroidManifestContent = $@"
   <application android:label=""App"">
-    <activity android:name="".TestActivity"">
+    <activity android:name="".TestActivity"" android:exported=""true"">
       <intent-filter>
         <action android:name=""android.intent.action.MAIN"" />
         <category android:name=""android.intent.category.LAUNCHER"" />

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Android/AndroidGradleProject.cs

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.IO;
 
+using Xamarin.Android.Tools;
 
 namespace Xamarin.ProjectTools
 {
@@ -16,13 +17,13 @@ public class AndroidGradleProject
 		/// <summary>
 		/// Android Gradle Plugin version (e.g., "8.5.0", "9.0.0")
 		/// </summary>
-		public string AgpVersion { get; set; } = "8.5.0";
+		public string AgpVersion { get; set; } = "9.1.0";
 
 		/// <summary>
 		/// Gradle wrapper version to use (e.g., "8.12", "9.0").
-		/// If null or empty, the Gradle wrapper version generated by gradle init is used.
+		/// Defaults to "9.3.1" (minimum required by AGP 9.1.0). If set to null or empty, the Gradle wrapper version generated by gradle init is used.
 		/// </summary>
-		public string? GradleVersion { get; set; }
+		public string? GradleVersion { get; set; } = "9.3.1";
 
 		GradleCLI gradleCLI = new GradleCLI ();
 
@@ -76,14 +77,15 @@ public static AndroidGradleProject CreateDefault (string projectDir, bool isAppl
 		/// <summary>
 		/// Creates a default Gradle project with specified AGP and Gradle versions.
 		/// </summary>
-		public static AndroidGradleProject CreateDefault (string projectDir, string agpVersion, string? gradleVersion, bool isApplication = false)
+		public static AndroidGradleProject CreateDefault (string projectDir, string agpVersion, string? gradleVersion, bool isApplication = false, int? compileSdk = null)
 		{
 			var proj = new AndroidGradleProject (projectDir) {
 				AgpVersion = agpVersion,
 				GradleVersion = gradleVersion,
 				Modules = {
 					new AndroidGradleModule (Path.Combine (projectDir, "TestModule")) {
 						IsApplication = isApplication,
+						CompileSdk = compileSdk ?? XABuildConfig.AndroidDefaultTargetDotnetApiLevel.Major,
 					},
 				},
 			};