Add AVD skin enumeration API

## 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> ListAvdSkinsAsync(
CancellationToken cancellationToken = default);
`

This would scan:
- `\/skins/` for standalone skins
- System image directories for bundled skins

## Related

- dotnet/android-tools#321 (device profile enumeration)