Simplify tile dismissal

We want floating tiles to be activatable, but also dismiss the effect by
clicking on the whitespace.

The current solution doesn't work as a floating tile that is in turn
split will pull focus from the children meaning those tiles can't be deleted.

Rather than special casing and z-manipulation we can simplify everything
by moving tile dismissal to the root of the effect and appropariately
propagating the event.

BUG: 465740


(cherry picked from commit 7b46f101)