Updated bubble bar position to be center aligned with the hotseat

Added a method to the device profile to calculate the vertical center of
the hotseat icons. Simplified the logic for positioning the bubble bar.

Test: TransientBubbleStashControllerTest
Test: PersistentBubbleStashControllerTest
Test: Visual. Go to home page, check that bubble bar is vertically
center aligned with the hotseat
Bug: 345491493
Flag: com.android.wm.shell.enable_bubble_bar

Change-Id: I52f1b94de79f6c912f43a88fcc5c884e20e56310