Dynamic WebView display breaks fullscreen mode on Android since Qt 5.9 (regression)

This issue was not present in 5.8.0, this looks to be a regression.

Testcase with `visible`:

import QtQuick 2.3
import QtQuick.Window 2.2
import QtWebView 1.1

Window {
    visible: true
    visibility: Window.FullScreen

    WebView {
        id: webview
        anchors.fill: parent
        url: 'http://example.com/'
        visible: false //true
    }

    Component.onCompleted: webview.visible = true
}

Testcase with `Loader`:

import QtQuick 2.3
import QtQuick.Window 2.2
import QtWebView 1.1

Window {
    visible: true
    visibility: Window.FullScreen

    Component {
        id: component
        WebView {
          anchors.fill: parent
          url: 'http://example.com/'
        }
    }

    Loader {
        id: loader
        active: false //true
        anchors.fill: parent
        sourceComponent: component
    }
    Component.onCompleted: loader.active = true
}

Expected behaviour: fullscreen mode on Android, no status/navigation/title bars visible.
Observed behavior: status/navigation bars visible on Android.

The issue is present only when WebView is shown dynamically, either with a Loader or by delayed `visible` toggling.
Those are pretty common usecases, so this basically breaks fullscreen apps with WebViews on Android.

When the webview gets hidden/removed — the application enters fullscreen mode, when it gets shown/loaded again — the application switches out from the fullscreen mode, showing the status/navigation bars and having the appication window resized.

All of those usecases were working fine on Qt 5.8.0 with the exact same code and devices / Android versions.