Only stash bubbles in overview when it's transient taskbar

Bug: 284104811
Test: manual - enable 3 button nav
             - expand a bubble in overview
             => observe that scrim is shown on top of taskbar and it
                isn't stashed
             - repeat with transient taskbar but instead it stashes
               when bubbles are expanded in overview
Change-Id: I725513bb0015c85f26fb1eedbe22da5eb2612e55