Fix NRT annotations in src\Xamarin.Android.Build.Tasks\Utilities following repository guidelines

This PR systematically updates nullable reference type annotations in the Utilities directory to follow the established patterns outlined in copilot-instructions.md.

Plan

Phase 1: Convert #nullable disable to #nullable enable (5 files at a time)

• ApplicationConfigNativeAssemblyGenerator.cs - ✅ Complex file with multiple nullable patterns fixed
• ApplicationConfigNativeAssemblyGeneratorCLR.cs - ✅ Complex file with similar patterns fixed
• CompressedAssembliesNativeAssemblyGenerator.cs - ✅ Simpler file with standard nullable patterns fixed
• JniRemappingAssemblyGenerator.cs
• ManagedMarshalMethodsLookupGenerator.cs

Phase 2: Update MSBuild Task property patterns

• ManagedResourceParser.cs
• ManifestDocument.cs
• ManifestDocumentElement.cs
• MarshalMethodsClassifier.cs
• MarshalMethodsNativeAssemblyGenerator.cs

Phase 3: Update helper and utility classes

• MonoAndroidHelper.cs
• TypeMapGenerator.cs
• TypeMappingDebugNativeAssemblyGenerator.cs
• TypeMappingReleaseNativeAssemblyGenerator.cs
• TypeMappingReleaseNativeAssemblyGeneratorCLR.cs

Pattern Changes Applied:

• Convert #nullable disable to #nullable enable
• Added default values for required string properties: string Property { get; set; } = ""
• Added default values for collection properties: List<ITaskItem> Items { get; set; } = []
• Made optional string/reference type fields nullable: string? field, AssemblyDescriptor? descriptor, StructureInfo? info
• Added null coalescing for nullable parameters: param ?? defaultValue
• Added explicit null checks where the null-forgiving operator would be inappropriate
• Protected against null Instance properties in sort comparisons
• Added null conditional operators where appropriate: Instance?.Property ?? defaultValue

Context #10326.

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