Make sure to require permissions (#1651)

If the permissions were denied, then throw.
Fixes  #1649

Xamarin.Essentials/FilePicker/FilePicker.android.cs

@@ -13,7 +13,7 @@ static async Task<IEnumerable<FileResult>> PlatformPickAsync(PickOptions options
         {
             // we only need the permission when accessing the file, but it's more natural
             // to ask the user first, then show the picker.
-            await Permissions.RequestAsync<Permissions.StorageRead>();
+            await Permissions.EnsureGrantedAsync<Permissions.StorageRead>();
 
             // Essentials supports >= API 19 where this action is available
             var action = Intent.ActionOpenDocument;