Update DispatcherHelpers

• Use CoreApplication.MainView.Dispatcher instead of CoreApplication.MainView.CoreWindow.Dispatcher in DispatcherHelpers for scenarios like background activation (valid dispatcher is on the CoreApplicationView, not necessarily on the CoreWindow).
• You can now pass an optional dispatcher when fetching images using FetchEncodedBitmapImageAsync and FetchDecodedBitmapImageAsync methods (helpful in multi-window scenario).