Set the offset of the dim layer of DisplayArea.

Bug: 217637411
Test: Check if the dim layer of DisplayArea is positioned correctly.
Change-Id: I8fd3494a3cd7ad6183ce6aa442e49f85b441c73f

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

@@ -652,7 +652,9 @@ public class DisplayArea<T extends WindowContainer> extends WindowContainer<T> {
         void prepareSurfaces() {
             mDimmer.resetDimStates();
             super.prepareSurfaces();
+            // Bounds need to be relative, as the dim layer is a child.
             getBounds(mTmpDimBoundsRect);
+            mTmpDimBoundsRect.offsetTo(0 /* newLeft */, 0 /* newTop */);
 
             // If SystemUI is dragging for recents, we want to reset the dim state so any dim layer
             // on the display level fades out.