Repository: flutter
Issue: [webview_flutter] : Route with WebView page is shown below the previous route when using SurfaceAndroidWebView()
Labels: platform-android,engine,a: existing-apps,a: platform-views,p: webview,package,has reproducible steps,P2,found in release: 2.2,found in release: 2.5,team-android,triaged-android

I tried to use SurfaceAndroidWebView() as below:
```
void initState() {
    super.initState();
    if (Platform.isAndroid) WebView.platform = SurfaceAndroidWebView();
}
```

The result is the new route with WebView is launched below the previous route. Even the WebView is underneath the previous page, WebView still can take the gesture to scroll. The button in that page can take action as well.

I can only repro this issue in these conditions:
1. Using **_add2app_**.
2. Have to use **_cached FlutterEngine_**.
3. Using FlutterFragment with **_RenderMode.surface_**.

Here is the screenshot that shows the issue:

![image](https://user-images.githubusercontent.com/44383755/96212084-c7cbd700-0f2a-11eb-82e6-849e8771ffd8.png)


When using **_RenderMode.texture_** no issue:

![image](https://user-images.githubusercontent.com/44383755/96211947-83d8d200-0f2a-11eb-854f-d8808d2839d3.png)

Here is the sample add2app project with the Flutter module:

[WebViewIssue_final.zip](https://github.com/flutter/flutter/files/5389221/WebViewIssue_final.zip)

Please open the project **_MyApplication2_** to build.

```
flutter doctor -v:

[✓] Flutter (Channel stable, 1.22.1, on Mac OS X 10.15.7 19H2, locale en-US)
    • Flutter version 1.22.1 at /Users/shihchang.hsiung/flutter-install/1.22.1-stable
    • Framework revision f30b7f4db9 (8 days ago), 2020-10-08 10:06:30 -0700
    • Engine revision 75bef9f6c8
    • Dart version 2.10.1

 
[✓] Android toolchain - develop for Android devices (Android SDK version 29.0.2)
    • Android SDK at /Users/shihchang.hsiung/Library/Android/sdk/
    • Platform android-29, build-tools 29.0.2
    • ANDROID_HOME = /Users/shihchang.hsiung/Library/Android/sdk/
    • ANDROID_SDK_ROOT = /Users/shihchang.hsiung/Library/Android/sdk/
    • Java binary at: /Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6222593)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 11.6)
    • Xcode at /Applications/Xcode11-6.app/Contents/Developer
    • Xcode 11.6, Build version 11E708
    • CocoaPods version 1.8.4

[✓] Android Studio (version 4.0)
    • Android Studio at /Applications/Android Studio.app/Contents
    • Flutter plugin version 49.0.2
    • Dart plugin version 193.7547
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6222593)

[✓] VS Code (version 1.50.0)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.15.0

[✓] Connected device (3 available)
    • Pixel XL (mobile)            • HT6C10201150                             • android-arm64 • Android 10 (API 29)
    • Shih-Chang’s iPhone (mobile) • 83c3a17190068bb7d7ff580eb58f916ab8e7db37 • ios           • iOS 13.6.1
    • iPhone X (mobile)            • F58DF26F-1527-4EDF-9AF5-57108B4BEFD8     • ios           • com.apple.CoreSimulator.SimRuntime.iOS-12-2 (simulator)

• No issues found!


```