Fix hidden Win32 TrayIcon appearing after icon updates

[Linlccc]

What does the pull request do?

Preserves the current Win32 tray icon registration state when its icon is updated.

A hidden TrayIcon now remains hidden until SetIsVisible(true) is called.

What is the current behavior?

TrayIconImpl.SetIcon calls UpdateIcon() without considering whether the tray icon is currently registered.

When _iconAdded is false, UpdateIcon() executes Shell_NotifyIcon(NIM.ADD, ...), causing a hidden or initializing TrayIcon to unexpectedly appear in the Windows system tray.

For example, updating the icon after hiding it can make it visible again:

trayIcon.IsVisible = false;
trayIcon.Icon = anotherIcon;

What is the updated/expected behavior with this PR?

Updating the icon preserves the current registration state:

• A hidden tray icon remains hidden.
• The tray icon is added only when SetIsVisible(true) is called.

To test the change:

1. Create and attach a TrayIcon.
2. Set IsVisible to false.
3. Change its Icon.
4. Verify that it does not appear in the Windows system tray.
5. Set IsVisible to true and verify that it appears.

How was the solution implemented (if it's not obvious)?

UpdateIcon(!_iconAdded);

When the tray icon is already registered, it is updated normally. When it is not registered, updating its icon does not add it to the system tray.

[avaloniaui-bot]

You can test this PR using the following package version. 12.2.999-cibuild0069313-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[MrJul]

LGTM!