Skip a step when blurring

backgroundBlurRadius blurs the entire content behind the current layer.
The way we implement it is to draw into an off-screen and use that as
the source for the blur. But we draw the unblurred off-screen first.
This isn't necessary.

Bug: 206674290
Test: manual: notification shade, SilkFX app
Change-Id: I4caa7ef3b5357c723fdc81c8e18b159c87366b60