Migrate GenerateProguardConfiguration from ILLink step to MSBuild task

[sbomer]

The new task runs AfterTargets="ILLink" and uses System.Reflection.Metadata instead of Mono.Cecil to scan linked assemblies.

[jonathanpeppers]

Looking at the test failures, I think this might have broken Proguard rules on NativeAOT:

I'll rerun to verify.

[Copilot]

Pull request overview

This PR migrates the GenerateProguardConfiguration functionality from an ILLink custom step (using Mono.Cecil) to an MSBuild task (using System.Reflection.Metadata). The task runs after the ILLink target and scans linked assemblies to generate ProGuard configuration rules for Android callable wrapper (ACW) types.

Changes:

• Added new MSBuild task GenerateProguardConfiguration using System.Reflection.Metadata for assembly inspection
• Updated MSBuild targets to invoke the new task after ILLink instead of as a linker custom step
• Removed the old ILLink step implementation that used Mono.Cecil

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 6 comments.

File	Description
src/Xamarin.Android.Build.Tasks/Tasks/GenerateProguardConfiguration.cs	New MSBuild task that scans linked assemblies for RegisterAttribute and generates ProGuard keep rules
src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.ILLink.targets	Added targets to prepare linked assemblies and invoke the new task; removed old linker custom step configuration
src/Microsoft.Android.Sdk.ILLink/GenerateProguardConfiguration.cs	Deleted old ILLink step implementation

[Copilot]

Path.GetDirectoryName can return null when the path doesn't contain directory information. The variable 'dir' should be checked for null before being passed to Directory.Exists and Directory.CreateDirectory. Consider using a pattern like the one in KeyTool.cs line 42 where the result is checked with IsNullOrEmpty before use.

Suggested change

	if (!Directory.Exists (dir))
	Directory.CreateDirectory (dir);
	if (!dir.IsNullOrEmpty () && !Directory.Exists (dir)) {
	Directory.CreateDirectory (dir);
	}

[Copilot]

This assignment to jargs is useless, since its value is never read.

Suggested change

	args.FixedArguments[1].Value is string jargs) {
	args.FixedArguments[1].Value is string) {