Illustrations for empty overflow (light/dark modes)

onResume, BubbleOverflowActivity lazy-updates UI theming

The stack has to collapse for the user to make theming changes, so
onResume is guaranteed to run before the user sees the overflow again.

Fixes: 148879187
Fixes: 149711636
Test: manual: toggle light/dark mode -> illustration updates

Change-Id: Iaad6c92d140e141ca9203990efc07df16cc7edf8