Always dismiss visible top associated overlay

Before, we launch the new activity to the existing overlay if its
associated activity and tag are exactly the same as the existing one.
Now we always dismiss the existing one because an overlay container
can only contain an activity.

Test: atest OverlayPresentationTest
Fixes: 343307330
Flag: EXEMPT bugfix

Change-Id: Id4d42eeb7077e48a18331cdbd3b063f089955e2d