vp9-noise estimate: Adhust threshold for Low level detection.

Change-Id: I1c78255c81cb4ac218964c80f8ecaf29f95b875b

vp9/encoder/vp9_noise_estimate.c

@@ -91,7 +91,7 @@ NOISE_LEVEL vp9_noise_estimate_extract_level(NOISE_ESTIMATE *const ne) {
   } else {
     if (ne->value > ne->thresh)
       noise_level = kMedium;
-    else if (ne->value > ((5 * ne->thresh) >> 3))
+    else if (ne->value > ((9 * ne->thresh) >> 4))
       noise_level = kLow;
     else
       noise_level = kLowLow;