Migrate WarnOnPreserveAttribute from ILLink step to MSBuild task

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

@@ -10,6 +10,7 @@ _ResolveAssemblies MSBuild target.
 
 <Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
 
+  <UsingTask TaskName="Xamarin.Android.Tasks.CheckForObsoletePreserveAttribute" AssemblyFile="$(_XamarinAndroidBuildTasksAssembly)" />
   <UsingTask TaskName="Xamarin.Android.Tasks.ProcessAssemblies"      AssemblyFile="$(_XamarinAndroidBuildTasksAssembly)" />
   <UsingTask TaskName="Xamarin.Android.Tasks.ProcessNativeLibraries" AssemblyFile="$(_XamarinAndroidBuildTasksAssembly)" />
   <UsingTask TaskName="Xamarin.Android.Tasks.ProcessRuntimePackLibraryDirectories" AssemblyFile="$(_XamarinAndroidBuildTasksAssembly)" />
@@ -226,6 +227,10 @@ _ResolveAssemblies MSBuild target.
     </ItemGroup>
   </Target>
 
+  <Target Name="_CheckForObsoletePreserveAttribute" AfterTargets="_PrepareAssemblies">
+    <CheckForObsoletePreserveAttribute Assemblies="@(_ResolvedUserAssemblies)" />
+  </Target>
+
   <Target Name="_IncludeNativeSystemLibraries">
     <PropertyGroup>
       <_AndroidIncludeSystemGlobalizationNative Condition=" '$(_AndroidIncludeSystemGlobalizationNative)' == '' ">true</_AndroidIncludeSystemGlobalizationNative>

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

@@ -39,10 +39,6 @@ This file contains the .NET 5-specific targets to customize ILLink
           Value="$(_ProguardProjectConfiguration)"
       />
       <_TrimmerCustomData Include="SystemIOHashingAssemblyPath" Value="$(_SystemIOHashingAssemblyPath)" />
-      <_TrimmerCustomSteps
-        Include="$(_AndroidLinkerCustomStepAssembly)"
-        BeforeStep="MarkStep"
-        Type="Microsoft.Android.Sdk.ILLink.WarnOnPreserveAttribute" />
 
       <!--
         Used for the <ILLink CustomSteps="@(_TrimmerCustomSteps)" /> value:

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

@@ -0,0 +1,62 @@
+#nullable enable
+
+using System;
+using System.IO;
+using System.Reflection.Metadata;
+using System.Reflection.PortableExecutable;
+using Microsoft.Build.Framework;
+using Microsoft.Android.Build.Tasks;
+
+namespace Xamarin.Android.Tasks
+{
+	/// <summary>
+	/// This task provides warnings when an assembly references the obsolete PreserveAttribute.
+	/// The PreserveAttribute used to indicate to the linker that a type or member should not be trimmed.
+	/// It had similar functionality to the newer DynamicDependencyAttribute, but was Android-specific and is now obsolete.
+	/// </summary>
+	public class CheckForObsoletePreserveAttribute : AndroidTask
+	{
+		public override string TaskPrefix => "COPA";
+
+		public ITaskItem[] Assemblies { get; set; } = [];
+
+		public override bool RunTask ()
+		{
+			foreach (var assembly in Assemblies) {
+				if (HasObsoletePreserveAttribute (assembly.ItemSpec, out string? assemblyName)) {
+					Log.LogCodedWarning ("IL6001", $"Assembly '{assemblyName}' contains reference to obsolete attribute 'Android.Runtime.PreserveAttribute'. Members with this attribute may be trimmed. Please use System.Diagnostics.CodeAnalysis.DynamicDependencyAttribute instead");
+				}
+			}
+			return true; // Warnings don't fail the build
+		}
+
+		bool HasObsoletePreserveAttribute (string assemblyPath, out string? assemblyName)
+		{
+			assemblyName = null;
+
+			try {
+				using var stream = File.OpenRead (assemblyPath);
+				using var pe = new PEReader (stream);
+
+				if (!pe.HasMetadata)
+					return false;
+
+				var reader = pe.GetMetadataReader ();
+				assemblyName = reader.GetString (reader.GetAssemblyDefinition ().Name);
+
+				foreach (var handle in reader.TypeReferences) {
+					var typeRef = reader.GetTypeReference (handle);
+					var ns = reader.GetString (typeRef.Namespace);
+					var name = reader.GetString (typeRef.Name);
+
+					if (ns == "Android.Runtime" && name == "PreserveAttribute")
+						return true;
+				}
+				return false;
+			} catch (BadImageFormatException ex) {
+				Log.LogDebugMessage ($"Could not read assembly '{assemblyPath}': {ex.Message}");
+				return false;
+			}
+		}
+	}
+}