Fix crash when overflow is selected and last bubble is removed

When the last bubble is removed we clean up the views associated with
the bubbles, including the overflow view.

BubbleOverflow#getExpandedView would create the view if it didn't exist
but it wouldn't be initialized which means it wouldn't have the Bubble
Positioner set on it.

When a bubble is removed some code runs that would do stuff to the
last expanded view, if it's non-null. If the overflow was the last bubble
expanded, rather than skipping that code because the expandedView is
null (because all the bubbles are gone), it was actually recreating
the overflow expandedView but it wouldn't be initialized at that time
resulting in the NPE.

This fixes the issue by modifying BubbleOverflow's getExpandedView to
only return the view, not do any view creation.

Additionally this change removes some redundant code in BubbleStackView.

Bug: 214399371
Test: atest BubbleOverflowTest
Change-Id: Ib8d94fc7ea1163e79146e1439ec3f404d3caa303