Modify Taskbar code to accommodate bubble bar gesture to stash/unstash

* Bubble bar typically follows the behavior of taskbar - if taskbar
  is shown, the bubble bar is shown, if taskbar hides, bubble bar
  hides.
* The bubble bar has 3 states: stashed, collapsed (unstashed but
  the bubbles are not expanded), and expanded. When bubbles are
  expanded, this means WMShell is rendering the bubble
  expanded view. In this situation taskbar becomes collapsed.

Bug: 253318833
Test: manual, with other CLs, see go/bubble-bar-tests
Flag: WM_BUBBLE_BAR
Change-Id: Ic210c382e7482c259ae543a0dc083fe9305cbf5b