Fix overlay below activity is dismissed

When launching a new overlay, the overlay container below an activity
could be dismissed unexpectedly because we check if the overlay
container is the top TF of the task, but the top TF could also be
occluded by fullscreen activity.

This CL checks if the overlay is the top child of its parent container
instead.

Fixes: 339195872
Test: atest OverlayPresentationTest
Flag: EXEMPT bugfix
Change-Id: I25a31f64ff006895a97d09f7fb5a583bdec19f11