Properly set the center of the circular mask.

The new circular mask did not take into account devices with a chin,
setting the center of the circular mask higher than it should be.

BUG: 32418552
Change-Id: Ia45e197492a57341ad7db620b1d3fcc4820eabf0

services/core/java/com/android/server/wm/CircularDisplayMask.java

@@ -242,7 +242,7 @@ class CircularDisplayMask {
         }
 
         double cx = (maskWidth - 1.0) / 2.0;
-        double cy = (maskHeight - 1.0) / 2.0;
+        double cy = (maskHeight - 1.0 + mScreenOffset) / 2.0;
         double radius = maskWidth / 2.0;
         int[] pixels = new int[maskWidth * maskHeight];