Add background blur in WindowManager

The CL adds background blur to the WindowManager LayoutParams as a
hidden API. The background blur is then passed on to the SurfaceControl
in WindowStateAnimator.prepareSurfaces.

Bug: 167166562
Test: m && flash && start app that requests background blur && verify it
works

Change-Id: I5dad3d8d5c75e1feff8fa035452dfe5c90efaa88