Reset the expanded bubble after dismissing the stack

This fixes the behavior where the stack stays expanded after swiping home after restoring the last bubble.
The problem here is that when the stack is dismissed, mExpandedBubble still points to the last selected bubble.
When the stack is restored, with the same bubble, the stack thinks that the expanded bubble didn't change, and does not update the animation controller correctly.
The controller still points to an old expanded view, which prevents the collapsing animation from running.

Fixes: 289988596
Flag: NONE
Test: Manual
     - Create bubble
     - Dismiss the stack
     - Create the same bubble and expand it
     - Swipe home
     - Observe that the stack collapses correctly
Change-Id: Ib5a51b67f319bd32b69faf7006bd31a34d8bdcbf