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 - flag turned on - see go/bubble-bar-tests
Test: manual - flag turned off:
             - launch an app, ensure taskbar is stashed
             - unstash taskbar, drag down on taskbar, ensure it
               becomes stashed
             - be in an app, unstash taskbar, interact with the app,
               ensure taskbar is stashed
             - be in an app, unstash taskbar, select an app from it,
               ensure taskbar is stashed & app is opened
Flag: WM_BUBBLE_BAR
Change-Id: I7b481d768182c8429160ab4a9b213b885a7d78bc