Refactor expanded view access in BubbleStackView

Create a method to access the expanded view that first checks if the
expanded bubble is null or not.
Also assign the expanded view to a variable in places it is used after
null check.

Bug: 342694122
Flag: EXEMPT refactor
Test: m
Test: manual test to expand and collapse bubbles
Change-Id: I9181b53537ebf3d14b5cefaf6265bbd2e557dd36