Add AVD skin enumeration API #12079

Summary

Add a method to list available Android emulator skins from the SDK's skins/ directory and system image skin directories.

Motivation

When creating AVDs programmatically, consumers need to present available skins to the user. Currently there's no API in Xamarin.Android.Tools.AndroidSdk to enumerate skins — consumers must manually scan \/skins/ and system image directories.

This is needed by tools like MAUI Sherpa and IDE extensions that provide a GUI for AVD creation with skin selection.

Proposed API

csharp public Task<IReadOnlyList<string>> ListAvdSkinsAsync( CancellationToken cancellationToken = default);

This would scan:

• \/skins/ for standalone skins
• System image directories for bundled skins

Related

• Add AvdManagerRunner.ListDeviceProfilesAsync() for AVD device profile enumeration android-tools#321 (device profile enumeration)