[webview_flutter_android] Fix race condition in flaky test (#5037)

The test was retrieving the current url directly after calling `loadUrl`. This seems to have a good chance to fail since loading a url isn't instantaneous. This changes the test to wait for the url to change.

Author: bparrishMines

packages/webview_flutter/webview_flutter_android/example/integration_test/legacy/webview_flutter_test.dart

@@ -248,11 +248,9 @@ Future<void> main() async {
       'test_header': 'flutter_test_header'
     };
     await controller.loadUrl(headersUrl, headers: headers);
-    final String? currentUrl = await controller.currentUrl();
-    expect(currentUrl, headersUrl);
 
-    await pageStarts.stream.firstWhere((String url) => url == currentUrl);
-    await pageLoads.stream.firstWhere((String url) => url == currentUrl);
+    await pageStarts.stream.firstWhere((String url) => url == headersUrl);
+    await pageLoads.stream.firstWhere((String url) => url == headersUrl);
 
     final String content = await controller
         .runJavascriptReturningResult('document.documentElement.innerText');