Flash when making a video fullscreen

# Bug Report

## Problem
I get a flash of the DOM content when making a video fullscreen.

### What is expected to happen?
The video becomes full screen without a flash of any DOM content.

### What does actually happen?
If you watch the "HEADER" text at the top of the screen in this video, you will see a brief flash just before the video goes fullscreen. It's right around the 3s mark.

https://user-images.githubusercontent.com/28525986/123274711-ab415f00-d4fb-11eb-98eb-0b31a1e74e80.mp4

If the flash seems slight or insignificant, that's because this was recorded using a sample app with a relatively light DOM. The app where this problem happens has a much heavier DOM and the problem is much more pronounced.

## Information

I created a sample app that shows the problem, you can download it [here](https://github.com/CodeWithOz/cordova-bad-web-workers/actions/runs/968151594) (the build artifact at the bottom of the page).

### Command or Code

Tap the video to start playing it, then just wait for 2-3 seconds and it will become fullscreen. You should see the "HEADER" text flash just before the video becomes fullscreen.

### Environment, Platform, Device

- cordova android: versions 9 and 10
- device: Tecno Pop2 Plus

### Version information

- cordova CLI: 10
- plugins:
- cordova-plugin-whitelist@1.3.4
- cordova-plugin-androidx
- cordova-plugin-androidx-adapter
- cordova-plugin-ionic-webview@4.2.1 (in my real app I'm using v5 of the webview plugin and the problem still happens as I mentioned above)

## Checklist

- [x] I searched for existing GitHub issues
- [x] I updated all Cordova tooling to most recent version
- [x] I included all the necessary information above

Start by running the linked sample app artifact and reproducing the flash after tapping the video and waiting 2–3 seconds. Then trace the Android fullscreen transition and WebView behavior; done means the video enters fullscreen without showing DOM content first, including with a heavy DOM.

Written by the indexing model from the issue text.