[Bug] Add better concept of Active vs Appearing as it relates to Modal Pages and the core app

### Description

The TabbedPage on Android needs to know when it's the "Active" page so that it can display the tabs on the main app container. When you navigate away from a TabbedPage to a page without tabs then the Tabs on the app container are hidden. Currently this is being tracked via "OnAppearing" but "OnAppearing" and "OnDisappearing" will also fire when a modal page is displayed or even in multi window scenarios when switching windows.

We need a better property to track if a page is active within the context of it's own container