Make the window root view transparent on the desktop

This CL adds a new config res to indicate whether the blurred
wallpaper is supported by device. This config is true by default and
is overridden to false on desktop.

In this CL, when the aforementioned config is false, the surface
is transparent.

Bug: 399112904
Flag: EXEMPT bugfix
Change-Id: I3b6310b086e3a26da1e7803b44727d9bad992898