MediaManager is not stopped and disposed on PopAsync

## 💬 Questions and Help
Hi,

I have a simple project (tried it with Shell also), with a MainPage calling a VideoPage with Navigation.PushAsync(new VideoPage()).
On the VideoPage, the media is played correctly. If I press back button (or call the Navigation.PopAsync() manually on a button click...), I can hear the video still playing when the page is removed from stack.
I thought the MediaManager would be stopped and disposed when the page is removed from the stack. So what do I miss ?

Thanks !

Reproduce the report with MainPage navigating to VideoPage through Navigation.PushAsync, then leaving with Navigation.PopAsync or the back button. Start by tracing VideoPage and MediaManager lifecycle handling; done means playback has stopped and the media resources are disposed when VideoPage is removed from the navigation stack.

Written by the indexing model from the issue text.