Don't reset alpha after dragging to dismiss

After dragging the bubble bar to dismiss, don't reset its alpha
value.

Flag: com.android.wm.shell.enable_bubble_bar
Fixes: 359804931
Test: manual
      - Create a bubble in the bubble bar
      - Drag bubble bar to dismiss
      - Create another bubble
      - Observe the new bubble does not flicker
Change-Id: I8d56f71d38ec33acda8efade50735b59254d77d4