Animate the stash handle for new bubbles

The stash handle now animates out before the new bubble animates in, and animates back in after the new bubble is hidden.
There's still some additional polish needed :)

Demo: http://recall/-/bJtug1HhvXkkeA4MQvIaiP/z30Ob1rcDUkNEphLAQsgV

Flag: ACONFIG com.android.wm.shell.enable_bubble_bar DEVELOPMENT
Bug: 280605846
Test: atest BubbleBarViewAnimatorTest
Change-Id: I03449286b01ec96de9834e24a707652ddbe49fb0