Ensure the scrim layer above home task & wallpaper.

Sepcify a positive number which must larger than task layer, to ensure
the scrim layer must occludes home task and wallpaper. Otherwise any
reparent transaction without specify layer could affect the visual
order.

Flag: EXEMPT bugfix
Bug: 363927522
Test: verify the scrim layer above home and wallpaper while playing
back-to-home animation.

Change-Id: I558e29e0ebe72b8efb42cacc9cafd7869b09a40e