ContentDialog FlowDirection does not follow the XamlRoot.Content FlowDirection

### Describe the bug

ContentDialog FlowDirection does not follow the XamlRoot.Content FlowDirection
ContentDialog FlowDirection 并不跟随 XamlRoot.Content 的 FlowDirection

As shown in the figure, when ContentDialog.XamlRoot is mounted to Window.Content, when ContentDialog is displayed and the FlowDirection of Window.Content is RTL, The ContentDialog is unchanged
如图所示，ContentDialog.XamlRoot 挂载到 Window.Content 时，显示 ContentDialog 时，Window.Content 的 FlowDirection 为 RTL 时，ContentDialog 没有变化

### Steps to reproduce the bug

See describe

### Expected behavior

ContentDialog FlowDirection should follow the XamlRoot.Content FlowDirection
ContentDialog FlowDirection 应该跟随 XamlRoot.Content 的 FlowDirection

### Screenshots


### NuGet package version

WinUI 3 - Windows App SDK 1.6 Preview 1: 1.6.240807006-preview1

### Windows version

Windows 11 (22H2): Build 22621

### Additional context

Also exists in Windows App SDK 1.5 and UWP (Including WinUI 2)

No source file or test is named. Start by reproducing the issue with a ContentDialog mounted through XamlRoot to Window.Content while the window uses RTL, then trace the ContentDialog and XamlRoot flow-direction behavior. Done means the dialog follows the XamlRoot.Content FlowDirection, with regression coverage for RTL.

Written by the indexing model from the issue text.