Enable nullable reference types in WaitForAppDetection.cs

[Copilot]

Opts the shipped WaitForAppDetection MSBuild task into nullable reference types, making explicit the existing contract that its single field can legitimately be null. Follows the conversion of its sibling DetectIfAppWasUninstalled.cs (#11598), which deferred this file.

Changes

• Added #nullable enable as the first line of src/Xamarin.Android.Build.Debugging.Tasks/Tasks/WaitForAppDetection.cs.
• Marked getPackagesAsync nullable, reflecting that GetRegisteredTaskObjectAssemblyLocal<T> returns null when nothing is registered:

System.Threading.Tasks.Task<List<AndroidInstalledPackage>>? getPackagesAsync;

The existing if (getPackagesAsync == null) return; guard in RunTaskAsync () already covers the null case, so no further changes were needed. No null-forgiving (!) operator is used; the project targets netstandard2.0, so ArgumentNullException.ThrowIfNull is not applicable.