Lumen generates gradated shading even if the light is a constant color sky light

Lumen generates a gradated shading even if objects are lit by a 360-degree constant sky light.
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I looked into this and found that this comes from Spherical Harmonics integration. Changing r.Lumen.ScreenProbeGather.DiffuseIntegralMethod from Spherical Harmonics to importance sample BRDF to game developers fix the dark shading.

I guess it's due to fast approximation with SH and it shouldn't be a big issue in most cases. But a licensee asked us to fix it so I'd like to make this jira for the issue.

1. Open the attached project
2. You can see gradated shading on a sphere
3. Change r.Lumen.ScreenProbeGather.DiffuseIntegralMethod from Spherical Harmonics to importance sample BRDF
4. The change made the shading completely flat.

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