Fix regression where StaggeredWorkspaceAnim didn't respect animateScrim

Before, this happened to work because we skipped setting the scrim when
doing an atomic animation, but the atomic animation code has been
removed. Add an explicit SKIP_SCRIM config flag instead.

Test: swipe up from overview to home, ensure scrim animates nicely

Bug: 185411781
Change-Id: I7bc14a11d9d416cc7336ea29d21107dcdbdbf782