[Refactoring] AsyncCommand and MediaPlugin could be simplified.

MediaPlugin merged in [Xamarin.Essentials](https://docs.microsoft.com/en-us/xamarin/essentials/media-picker?tabs=ios).
And AsyncCommand supported in [XCT](https://github.com/xamarin/XamarinCommunityToolkit/blob/aba6205274a7fb8c8a55eaedb8001e3a693ed5f1/src/CommunityToolkit/Xamarin.CommunityToolkit/ObjectModel/AsyncCommand.shared.cs).