Gates blur code behind BlurUtils#supportsBlursOnWindows.

This "fixes" the bug where wallpaper zoom does not reset to 0 when
screen turns off since we no longer require a valid surface to set the depth
when blur is disabled.

Note that the bug still exists when blur is enabled, which will need to be
fixed in a follow up CL.

Bug: 157946272
Change-Id: I43179435885c95eb2ecf406fa5c291badf5a1ed3