Add a background scrim to launcher for text protection.

This is a temporary approach to dim the wallpaper. Flip on ENABLE_WALLPAPER_SCRIM to enable the scrim.

In the long term we'll dim the wallpaper layer directly.

Test: Manual
Bug: 185890335
Change-Id: Ia88597bbcba6ae1536015d1b2a35260cab4a4453