Store the flyout in BubbleBarBubble

Extracts the flyout message from the bundle and stores it in
BubbleBarBubble.

Flag: com.android.wm.shell.enable_bubble_bar
Bug: 277815200
Test: builds successfully -- code is not wired up yet
Change-Id: I5e810290991b4ee638f43add4d6a6514edb167af