Stash taskbar when bubbles is expanded in overview

Test: manual - have transient taskbar
             - have a bubble
             - go to overview
             - expand bubbles
             => observe that transient taskbar stashes
             - collapse bubbles
             => observe that transient taskbar unstashes

             - verify that expanding bubbles in app and on launcher
               home behaves as expected (stashes taskbar in app and
               don't see taskbar or stashed handle on home)
Bug: 284104811

Change-Id: I3d7057ed651e66ab2a0292725f30153ee4d6d51e