fix webview loading screen to cover the whole screen

[seblau]

With shoutem platform 2.1.0-rc3 and the new react-native-webview 5.12.1, the loading screen shows a white view, see:

For the issue see: https://github.com/react-native-community/react-native-webview/issues/684

I applied the default styling for the loading screen taken from here: https://github.com/react-native-community/react-native-webview/blob/master/src/WebView.styles.ts#L17