Fix an issue where sometimes we wouldn't animate while dismissing

When bubbles are expanded and we drag the currently selected bubble
to the dismiss target, we'd fade out the expanded view.

In the situation where it was the last bubble being removed, this
flag would not get reset. This means the next time the last bubble
is dragged out, it wouldn't get animated.

To fix this, when the last bubble gets animated out, reset this flag.

Bug: 279074302
Test: manual - have 1 bubble, expand it, drag to dismiss it
             - get another bubble, expand it, drag to dismiss it
               => while dragging, the expanded view should always
                  fade out
Change-Id: Ibc3ddaab7c48e285b7ed25c32711bef7ebdb0dc9