SystemBackdropElement does not handles theme changed event

### Describe the bug

SystemBackdropElement does not handle `RequestedTheme` changes at all from [the source code](https://github.com/microsoft/microsoft-ui-xaml/blob/winui3/main/src/controls/dev/SystemBackdropElement/SystemBackdropElement.cpp).

### Why is this important?

If the user toggles app theme at runtime, the backdrop is broken, causing a very bad readability.

### Steps to reproduce the bug

1. Open winui 3 gallery to SystemBackdropElement page
2. Toggle the control example theme

### Actual behavior

_No response_

### Expected behavior

_No response_

### Screenshots

### NuGet package version

2.0.12

### Windows version

Windows 10 (1809): Build 17763

### Additional context

I have [an almost identical implementation](https://github.com/HO-COOH/WinUIEssentials/blob/master/WinUI3Package/AcrylicVisual.cpp) that handles this correctly.

Start in src/controls/dev/SystemBackdropElement/SystemBackdropElement.cpp and trace how RequestedTheme changes are handled, using the SystemBackdropElement page in WinUI 3 Gallery to reproduce the issue. Verify that toggling the control example theme updates the backdrop correctly and preserves readability at runtime.

Written by the indexing model from the issue text.