Show ContextMenu on long press on visual element in mobile platform

### Description

We can now attach a context menu to any visual element using a MenuFlyout control, but on mobile it doesn't work correctly. It should appear on long press on visual element.

### Public API Changes

API is not changed.
### Intended Use-Case

It is very important issue, because context menu is used very often and developers wait that it will work on mobile on long press. This functionality will help .NET MAUI become #1 platform.

Start by tracing how MenuFlyout is attached to a visual element on the mobile platform and how long-press input is handled. Verify the behavior on mobile devices or emulators, then confirm that the context menu appears after a long press without changing the public API.

Written by the indexing model from the issue text.