Add a triangluar pointer to indicate the currently expanded bubble

Introduces BubbleExpandedViewContainer to wrap the currently expanded
view and render the triangle
   * This view should also handle rendering the header
   * ActivityView will live here too eventually maybe

Test: visual - enable bubbles, open a bubble, note the pointer
Bug: 111236845
Change-Id: I49569a0355eab29c9f1b6bb9abe1cd2d10b9b27b