Bubble bar flyout polish

Animate the elevation of the flyout along with the expansion animation.
Also start revealing the triangle as soon as we're past the bubble bar.

Flag: com.android.wm.shell.enable_bubble_bar
Bug: 277815200
Test: atest BubbleBarFlyoutViewScreenshotTest
Test: atest BubbleBarFlyoutControllerTest
Change-Id: Ie7e27240ae3ab86f8e7c3f39aa1c80885cfdc98e