[webview_flutter_android][webview_flutter_wkwebview] Fixes bug where PlatformWebViewWidget doesn't rebuild when the controller changes

[bparrishMines]

In a scenario where a WebViewWidget was updated with a new WebViewController, the native WebView from the new controller would not be shown. e.g.

 class WebViewExample extends StatefulWidget {
  const WebViewExample({super.key});

  @override
  State<WebViewExample> createState() => _WebViewExampleState();
}

class _WebViewExampleState extends State<WebViewExample> {
  late WebViewController controller;

  @override
  void initState() {
    super.initState();
    controller = WebViewController()
      ..loadRequest(Uri.parse('https://flutter.dev'));
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('Flutter Simple Example')),
      body: WebViewWidget(controller: controller),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          setState(() {
            controller = WebViewController()
              ..loadRequest(Uri.parse('https://google.com'));
          });
        },
        child: const Icon(Icons.add),
      ),
    );
  }
}                                                                                                                                                                                                    

From testing, the WebViewWidget would continue showing the original WebViewController and eventually freeze the PlatformView preventing button presses.

This is because the PlatformView widget only creates the native PlatformView once and doesn't recreate it when the creation parameters change (as expected). This adds a default key to the PlatformWebViewWidget that is used to indicate when the underlying widget needs to be removed and recreated. See https://api.flutter.dev/flutter/widgets/Widget/key.html

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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[mvanbeusekom]

LGTM

[stuartmorgan-g]

LGTM