Defer showing the expanded view for bubble bar

This change defers showing the expanded view for the bubble bar
until launcher requests it.

Flag: com.android.wm.shell.enable_bubble_bar
Fixes: 339683389
Test: manual
      - add new bubble that auto expands
      - observe expanded view shows when bubble bar starts expanding
Change-Id: I3f1f48a469b9c5c75ff2cae334d5c3c4edb65e9f