Fix SF blur to be as blurry as Gaussian blur

The current SF blur is using an approximation of Gaussian blur. The
blur radius has to be 3.2 times higher in order the create the same
effect that Gaussian blur would.
This CL corrects that, making the SF blur as blurry as a Gaussian blur.
This way, the effect that a certain blur radius creates with Android
cross-window blur resembles closely what Gaussian blur would do with the
same radius.

This also aligns the effect created by in-app blurs and cross-app
blurs with the same blur radius.

Bug: 192241610
Test: atest BlurTests
Test: manual, comparing in-app blur and cross-app blur
Change-Id: I5fe428ddf01dbe3d2bd6df18957165588848235e