Prepare for wiring bubble bar flyout

Implement BubbleBarFlyoutPositioner in BubbleBarViewController

Flag: com.android.wm.shell.enable_bubble_bar
Bug: 277815200
Test: builds successfully -- code is not wired up yet
Change-Id: Ib2d9b1f2f6e972f6ce949e10d50334a501c8c805
3 files changed