public void Download(string uri, string filename)
{
Android.Net.Uri contentUri = Android.Net.Uri.Parse(uri);
Android.App.DownloadManager.Request r = new Android.App.DownloadManager.Request(contentUri);
r.SetDestinationInExternalPublicDir(Android.OS.Environment.DirectoryDownloads, filename);
r.AllowScanningByMediaScanner();
r.SetShowRunningNotification(true);
r.SetNotificationVisibility(Android.App.DownloadVisibility.VisibleNotifyCompleted);
r.SetVisibleInDownloadsUi(true);
Android.App.DownloadManager dm = (Android.App.DownloadManager)Xamarin.Forms.Forms.Context.GetSystemService(Android.Content.Context.DownloadService);
// dm.NotifyAll();
dm.Enqueue(r);
}