Stash taskbar on swipe to go home for 3p fallback launcher.

Follow up to b/21916480, which was reverted due to test failures which are now fixed.

Fix: 269070986
Test: manual. Install launcher3WithoutQuickstepDebug and swipe to home from within an app.
Change-Id: I52fad88a25325e5c38a600ccf90b30ebf60bf71a