Remove unused using directives from Aapt2.cs

[Copilot]

Aapt2.cs carried several unused imports in its using block, which added noise and overstated the file's actual dependencies. This change trims the file to only the namespaces it references.

• Using cleanup

  • Removed six unused directives from src/Xamarin.Android.Build.Tasks/Tasks/Aapt2.cs
  • Preserved the existing ordering of the remaining directives

• Scope

  • No task logic, behavior, or signatures changed
  • No test code or project configuration changed

using System;
using System.IO;
using System.Threading;
using System.Text;
using Microsoft.Build.Utilities;
using Microsoft.Build.Framework;
using System.Collections.Generic;
using System.Collections.Concurrent;
using Xamarin.Android.Tools;
using Microsoft.Android.Build.Tasks;