Use leaf task as presentation host

With the introduction of desks, which are parent tasks of desktop
freeform windows, using the root task of a presentation host
activity is no longer valid (because the root task is a desk).

Flag: com.android.window.flags.enable_presentation_for_connected_displays
Bug: 418850221
Test: PresentationTest
Change-Id: Icb244618be643dfc2989e18f05ae0770a709fa65