Use globalOverlay as the contextOverlay in MobileContext

In DesktopContext we use localOverlay, but since inspectors
are full-screen on mobile, the localOverlay is actually
on top of the inspector. This caused link inspector suggestions
to not be shown.

Bug: 67399
Bug: 66753
Change-Id: I95883164797f6fb27659abf6445be98133c19581