Reuse existing paths to render overflow

Set expanded view visibility in one place
- Move logic from controller to stack view, which has access to overflow

Bug: 138116789
Test: manual - make 6 bubbles, overflow shows 1 bubble
Test: atest SystemUITests
Change-Id: I0dd832106303e5b4aa4621d6fdf6d2e949cbc155
2 files changed