Fix NavigationStarting & NavigationCompleted event for Avalonia.WebView.Android

[LazuliKao]

1. Fixed NavigationStarting and NavigationCompleted events on the Avalonia.WebView.Android (non-Blazor mode).
2. Implemented IWebViewControl.ExecuteScriptAsync for the Avalonia.WebView.Android.

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1. 修复安卓端（非Blazor模式）无法触发NavigationStarting和NavigationCompleted事件的bug
   • Clients\AvaloniaWebViewClient.cs移到Clients\Blazor\AvaloniaBlazorWebViewClient.cs，实现AvaloniaWebViewClient用于非Blazor模式的WebViewClient，相应对应事件
2. 实现安卓端的 IWebViewControl.ExecuteScriptAsync用TaskCompletionSource转成普通异步。

    var cts = new TaskCompletionSource<string?>();
      _webView.EvaluateJavascript(
          javaScript,
          new JavaScriptValueCallback(result => cts.TrySetResult(result?.ToString()))
      );
      return cts.Task;