[webview_flutter_android] Expose allowFileAccessFromFileURLs and allowUniversalAccessFromFileURLs

[jackxuechen]

This PR exposes Android WebView file URL access settings through AndroidWebViewController.

The following Android WebSettings APIs are added:

• setAllowFileAccessFromFileURLs
• setAllowUniversalAccessFromFileURLs

These settings allow applications loading local web content using file:// URLs to explicitly configure whether local resources and resources from other origins can be accessed.

This is useful for applications embedding local HTML assets that need to load additional local files or communicate with remote web services.

The APIs are only available on Android and do not affect other WebView implementations.

Issues

N/A

This is an API enhancement request to expose existing Android WebSettings functionality.

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Code Review

This pull request adds support for configuring file URL access permissions (setAllowFileAccessFromFileURLs and setAllowUniversalAccessFromFileURLs) in the Android WebView plugin. This includes updates to Pigeon definitions, generated Kotlin and Dart bindings, the controller implementation, and corresponding unit tests. The review feedback suggests replacing @NotNull with @NonNull in WebSettingsProxyApi.java to maintain consistency with the rest of the codebase.