[webview_flutter_wkwebview][webview_flutter_android] Fixes bug where the WebViews could not be released

[bparrishMines]

Fixes flutter/flutter#118579

tldr;
Fixed a bug caused by misusing withWeakRefenceTo. And fixed integration tests that started to stall because of the change.

So, apparently this causes a circular reference:

late final WKWebView _webView = withWeakRefenceTo(this, (
    WeakReference<WebKitWebViewController> weakReference,
  ) {
    return _webKitParams.webKitProxy.createWebView(
      _webKitParams._configuration,
      observeValue: (
        String keyPath,
        NSObject object,
        Map<NSKeyValueChangeKey, Object?> change,
      ) {
        // use weakReference
        }
      },
    );
  });

but this doesn't:

late final WKWebView _webView = _webKitParams.webKitProxy.createWebView(
    _webKitParams._configuration,
    observeValue: withWeakRefenceTo(this, (
      WeakReference<WebKitWebViewController> weakReference,
    ) {
      return (
        String keyPath,
        NSObject object,
        Map<NSKeyValueChangeKey, Object?> change,
      ) {
        // use weakReference
      };
    }),
    instanceManager: _webKitParams._instanceManager,
  );

It seems important that the callback method can't include any references to external values. I should have followed my own example 🤦‍♂️.

It also seems this change affected a bunch of integration tests. My assumption that Android required displaying the WebView to get the onPageFinished callback may have been incorrect. The callback doesn't work consistently unless a widget is loaded. This now happens on iOS AND Android. I was unable to debug why this is the case. It could just be quirk of integration tests. The callbacks still work in the example app when running in debug and release mode, so I don't think it is indicative of another bug.

---

Also tack on an additional quick fix for Android:

Fixes flutter/flutter#119332

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[arifje]

When will this fix be merged? Need it to get my project working 😁

[bparrishMines]

Making this nullable fixes flutter/flutter#119332

[cyanglaz]

LGTM