WebView in WebviewScaffold is always shown if onStateChanged with finishLoad is triggered

[cremor]

System info

Issue occurs on: Android for sure, haven't tested iOS
Plugin version: 0.3.7
Flutter doctor output:

[√] Flutter (Channel stable, v1.7.8+hotfix.4, on Microsoft Windows [Version 10.0.17763.678], locale de-AT)

[√] Android toolchain - develop for Android devices (Android SDK version 29.0.1)
[√] Android Studio (version 3.4)
[√] Connected device (3 available)

• No issues found!

Steps to Reproduce

1. Create an app that uses the WebviewScaffold with the hidden parameter to true.
2. Once the initial page load is finished (and the web view is shown), hide the web view by calling FlutterWebviewPlugin.hide().
3. Do one of the following:
   a. Have a page finish loading because you called hide() while it was loading.
   b. Call FlutterWebviewPlugin.reload().
4. Now the FlutterWebviewPlugin is shown again, although it should stay hidden.

I assume the problem is here:

flutter_webview_plugin/lib/src/webview_scaffold.dart

Lines 105 to 111 in c3b3fcf

	if (widget.hidden) {
	_onStateChanged =
	webviewReference.onStateChanged.listen((WebViewStateChanged state) {
	if (state.type == WebViewState.finishLoad) {
	webviewReference.show();
	}
	});

That event should be handled only once.

Logs

(No output from flutter run --verbose.)

flutter analyze:

No issues found! (ran in 4.3s)

[√] Flutter (Channel stable, v1.7.8+hotfix.4, on Microsoft Windows [Version 10.0.17763.678], locale de-AT)
    • Flutter version 1.7.8+hotfix.4 at C:\Android\flutter
    • Framework revision 20e59316b8 (4 weeks ago), 2019-07-18 20:04:33 -0700
    • Engine revision fee001c93f
    • Dart version 2.4.0


[√] Android toolchain - develop for Android devices (Android SDK version 29.0.1)
    • Android SDK at C:\Android\Sdk
    • Android NDK location not configured (optional; useful for native profiling support)
    • Platform android-29, build-tools 29.0.1
    • ANDROID_HOME = C:\Android\Sdk
    • Java binary at: C:\Program Files\Android\Android Studio\jre\bin\java
    • Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1343-b01)
    • All Android licenses accepted.

[√] Android Studio (version 3.4)
    • Android Studio at C:\Program Files\Android\Android Studio
    • Flutter plugin version 38.2.1
    • Dart plugin version 183.6270
    • Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1343-b01)

[√] Connected device (1 available)
    • Android SDK built for x86 • emulator-5554 • android-x86 • Android 9 (API 28) (emulator)

• No issues found!