Unable to open Split-Button flyout within a ListView using ALT + Down on the keyboard

### Describe the bug

According to this old thread https://github.com/microsoft/microsoft-ui-xaml/issues/1238 the correct way to open a the flyout from a split button using the keyboard is by using ALT + Down. However, when the Split-Button is in a ListView using ALT + Down moves focus from the Split-Button to the next item in the list instead of opening the Split-Button's flyout. Contrary to that information, for this scenario I'm able to open the Split-Button's menu flyout using SHIFT + ALT + Down. Can we get confirmation on whether this is intended behavior or if this is a bug?

See my test repo for a quick repro https://github.com/bbonaby/SplitButtonTestApp - Currently using WinAppSDK version 1.5.240802000

### Steps to reproduce the bug

- Create a Win UI 3 app with two or more split button controls inside a ListView.
- launch the app and Tab into the ListView and then tab into the Split-Button.
- Focus will be on the entire split button, then on the Keyboard type ALT + Down arrow, you'll notice that the focus now goes to the next item in the list instead of opening the flyout.

### Expected behavior

The flyout in the Split-Button opens when the Split-Button has focus and the user presses the ALT + Down keys

### Screenshots

Here is a video of me using the on screen keyboard to at first attempt to open the Split-Buttons flyout with ALT + Down. Then at the end I show that I can open it with SHIFT + ALT + Down.

https://github.com/user-attachments/assets/ee39e9e1-afae-40de-b3da-3dcaa14fa8eb

### NuGet package version

WinUI 3 - Windows App SDK 1.5.6: 1.5.240802000

### Windows version

Windows 11 (22H2): Build 22621

### Additional context

_No response_

Start with the linked SplitButtonTestApp repro using Windows App SDK 1.5.6, focusing on a SplitButton inside a ListView and its ALT+Down keyboard handling. Confirm the behavior against the expected interaction: ALT+Down should open the focused SplitButton flyout rather than move focus to the next list item.

Written by the indexing model from the issue text.