Audit static field usage: Make mutable static fields readonly to improve MSBuild safety

[Copilot]

This PR addresses static field usage in Xamarin.Android.Build.Tasks to improve MSBuild safety and reduce potential thread safety issues, as identified in the audit request.

Problem

The original issue identified 27 mutable static fields that could be problematic in MSBuild context where tasks might run in parallel or be cached inappropriately. Mutable static fields can cause:

1. Race conditions during parallel task execution
2. Unexpected state mutations between builds
3. Thread safety issues in IDE scenarios where SDK paths might change

Solution

After systematically auditing all static fields in the Build.Tasks assembly, I identified and fixed 13 static fields that should be immutable after initialization by adding the readonly modifier:

XML and Configuration Fields:

• ManifestDocument.AndroidXmlNamespace & AndroidXmlToolsNamespace - XML namespace constants
• ManifestDocument.androidNs & androidToolsNs - Cached namespace references
• JavaObjectsXmlFile.settings - XML writer settings
• SupportsGLTextureAttribute.mapping - Manifest mapping configuration

Regex and Parsing Fields:

• Aapt2Link.exraArgSplitRegEx - Compiled regex for argument parsing
• ResourceIdentifier.validIdentifier - Compiled regex for validation
• CheckForInvalidResourceFileNames.javaKeywords - Java keyword array
• RtxtParser.knownTypes - Set of known resource types

Utility Fields:

• ManagedResourceParser.compareTuple - Comparer instance
• RemoveUnknownFiles.IsWindows & PathUtil.IsWindows - Platform detection flags

Analysis of Remaining Static Fields

The remaining static fields were analyzed and found to be appropriate:

• MonoAndroidHelper.SupportedVersions & AndroidSdk - Set once per build by ResolveSdksTask following established MSBuild patterns
• AndroidLinkConfiguration.configurations - Uses ConditionalWeakTable which is already thread-safe
• MonoAndroidHelper.uname - Uses Lazy<T> with thread-safe initialization

Impact

These changes eliminate potential race conditions by making static fields immutable after initialization, while maintaining all existing functionality. The modifications are surgical, affecting only field declarations without changing any logic or behavior.

Fixes #5996.

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