Audio track flyout from `MediaTransportControls` still has transparent background

### Describe the bug

In release of Windows App SDK 1.6, it is claimed that "flyouts from `MediaTransportControls` have transparent background" had been fixed. But it seems Audio track selection flyout still has a transparent background.

### Steps to reproduce the bug

1. Create a `MediaPlayerElement` and set the source to a video with multiple audio tracks.
2. Click on the audio track selection button in the player's MTC.

### Expected behavior

_No response_

### Screenshots

_No response_

### NuGet package version

WinUI 3 - Windows App SDK 1.6.0: 1.6.240829007

### Windows version

Windows Insider Build (xxxxx)

### Additional context

_No response_

Start with MediaTransportControls and the audio track selection flyout used by a MediaPlayerElement with multiple audio tracks. Reproduce the transparent background on Windows App SDK 1.6.0, then compare it with the other MediaTransportControls flyouts that were reportedly fixed. Done means the audio track flyout has an opaque background without regressing the other flyouts.

Written by the indexing model from the issue text.