Make only top bubble clickable when stack is collapsed

Bug: 157934340
Test: atest SystemUITests
Test: manual, screen recording https://drive.google.com/file/d/12itDLsVQZrCQVYMjgJhuWlTslOJdyGqy/view?usp=sharing
Change-Id: I865a8d508965e88e78e516143915fd895d4f3782