Map controls on different pages are interfering with each other with MoveToRegion is invoked

[renfred]

Map controls on different pages are interfering with each other with MoveToRegion is invoked.

Steps to reproduce:

1. Add UnifiedMap control to two different pages
2. On each of the page, use MoveToRegion to set the map focus to different area
3. Switch between the two pages and you will notice that the map is interfering with each other

Investigation discovered that this is because MoveToRegion method uses MessagingCenter with a static "MessageMapMoveToRegion" internal const string.