`SplitButton` flyout is not honoring the `Flyout.Placement` property

### Describe the bug

The `Flyout` attached to a `SplitButton` is always opening with a `BottomEdgeAlignedLeft` placement.
We don't have any way to use a different placement (like `Bottom` to have the `Flyout` being centered).

### Why is this important?

In our application, we are using both split buttons and regular buttons to show some flyouts.
We'd like to have all our flyouts to be `Bottom` aligned for visual consistency.

### Steps to reproduce the bug

Use the following XAML:

```xml

```

### Actual behavior

The `Flyout` is always left aligned with the `SplitButton`:

### Expected behavior

The `Flyout` should use the `Flyout.Placement` property if set by the user.
When using `Placement="Bottom"` we're expecting the following layout:

### Screenshots

_No response_

### NuGet package version

WinUI 3 - Windows App SDK 1.8.5: 1.8.260209005

### Windows version

_No response_

### Additional context

_No response_