[Android] BackdropFilter has no effect on PlatformView widgets (WebView, YouTube players, etc.)

## Steps to reproduce

1. Clone the reproduction repository: https://github.com/AcarFurkan/flutter_video_blur_repro
2. Run the app on a physical device or emulator
3. Observe two examples:
- **video_player example**: Toggle between TextureView and PlatformView modes
- **youtube_player_flutter example**: Uses WebView (always a PlatformView)
4. Scroll content behind the AppBar and BottomNavigationBar which both use `BackdropFilter` with blur

## Demo

https://github.com/user-attachments/assets/540661b1-e196-4229-8892-7ef8320ccd1f

## Expected behavior

`BackdropFilter` should blur all content behind it, including PlatformView widgets, just like it blurs regular Flutter widgets.

## Actual behavior

- Flutter widgets (Container, Text, etc.) blur correctly.
- PlatformView-based content (WebView, native video views) passes through completely un-blurred.
- With `video_player`, switching to `VideoViewType.textureView` makes blur work, but `VideoViewType.platformView` does not blur.

## The problem

There is no workaround for packages that rely exclusively on PlatformView (e.g., `youtube_player_flutter`, `webview_flutter`). Unlike `video_player`, these
packages don't offer a TextureView alternative, so there is no way to achieve blur effects over their content.

This makes it impossible to build common UI patterns like frosted glass AppBars or blurred overlays in apps that display YouTube videos or web content.

Are there any plans or a roadmap to support BackdropFilter compositing with PlatformViews at the engine level? In the meantime, are there any recommended workarounds for packages that exclusively rely on PlatformView, such as webview_flutter?