Only show the first 2 bubbles in the stack when collapsed

Test: manual - add more than 2 bubbles, note you only see the top 2
             - expand & collapse the stack, note you only see top 2
               after collapse
             - fling the stack around, note once it hits the edge
               you only see top 2 bubbles but while dragging around
               you see all of them following
Bug: 180142824
Change-Id: I168a14e1fba1506f53ed0734b55b44971d128de4