Check bubble count for initial state animation

With optional overflow, we need to check for actual bubble count
before animating the bubble bar to its initial state.

Flag: com.android.wm.shell.enable_bubble_bar
Fixes: 359403793
Test: manual
       - create first bubble on home
       - observe bubble bar animates correctly to initial state
       - add second bubble
       - observe bubble bar bounces correctly and does not slide
         in from the bottom of the screen
Change-Id: I53a34bc00b24e8a6504bcca0532054755554b9ac