Don't animate in the bubble bar unless there are bubbles

Fixes: 290608658
Test: Manual:
      - Remove all bubbles
      - Lock and unlock device
      - Observe that bubble bar is not displayed

      - Remove all bubbles
      - Restart launcher process
      - Observe that bubble bar is not displayed

Change-Id: I850d307394230968f86abc23ba0b4e94f55e10f0