Application Error net::ERR_INSUFFICIENT_RESOURCES (file:///android_asset/www/index.html) on app startup

Hello,

I'm getting this error upon application startup since v2 of `cordova-plugin-ionic-webview` on some older Androids in about 1 of 5 startups (couldn't reproduce it on Oreo+ yet):

I've debugged a litte and the root cause of the problem seems to be that the new version intercepts the initial request to the `launchUrl` in `com.ionicframework.cordova.webview.IonicWebViewEngine#onPageStarted` and tries to load the `CDV_LOCAL_SERVER` Url instead. This defeats Cordova's own protection against errors during load in `org.apache.cordova.engine.SystemWebViewClient#onPageStarted` and `org.apache.cordova.engine.SystemWebViewClient#onPageFinished` that sets and unsets the `isCurrentlyLoading` flag that is used to swallow any errors during loading process in `org.apache.cordova.engine.SystemWebViewClient#onReceivedError`.

The call sequence in our case is (sometimes):
`org.apache.cordova.engine.SystemWebViewClient#onPageStarted` with `launchUrl`
`org.apache.cordova.engine.SystemWebViewClient#onPageStarted` with `CDV_LOCAL_SERVER`
`org.apache.cordova.engine.SystemWebViewClient#onPageFinished` with `launchUrl`
(after this step the `isCurrentlyLoading` flag is set to `false`)
`org.apache.cordova.engine.SystemWebViewClient#onReceivedError` with `launchUrl`
`org.apache.cordova.engine.SystemWebViewClient#onPageFinished` with `CDV_LOCAL_SERVER`

The fun part is that the app loads fine in the background but when the OK button is clicked in the dialog, Cordova closes the app.

Cordova: 8.0.0
Cordova-Android: 6.4.0 (tried with 7.1.1, too)
cordova-plugin-ionic-webview: 2.1.0

Start with com.ionicframework.cordova.webview.IonicWebViewEngine#onPageStarted and compare it with org.apache.cordova.engine.SystemWebViewClient#onPageStarted, #onPageFinished, and #onReceivedError. Reproduce the startup sequence on an older Android version and trace the isCurrentlyLoading flag. Done means the startup error is no longer shown and the app remains open when the load completes.

Written by the indexing model from the issue text.