Adds shadows and a default background color to Bubbles.

This makes the aliasing issues with clip path more apparent, but not inherently worse. We'll fix those issues in a subsequent CL, I just want to add this change now because transparent bubbles with no shadows are complicating stack-position debugging!

Test: manual
Bug: 123521453
Change-Id: I06a925ba6db64e92640146b9642396303b098f99