Improvement for the Bubble Bar Bubbles' Shadows

The issue was caused by the alpha animation applied to the bubble bar.
Added code to check if the alpha applied to the bubble bar is less than
1 and instruct child bubble views not to provide a shadow outline in
such cases. While this is not a full fix, but rather an improvement,
b/345490679 should introduce a proper fix.

Bug: 345484712
Test: visual - go to any application and stash/unstash bubble bar
Flag: com.android.wm.shell.enable_bubble_bar
Change-Id: Icb6bdb009f4d5998ec1638f97de89f7a4c9feccf