Add overrides and switch to nullptr keyword for all files

Adds remaining missing overrides and nullptr usages, missed due to
an extreme failure in tool usage.

Change-Id: I56abd72975a3999ad13330003c348db40f59aebf
diff --git a/libs/hwui/DeferredLayerUpdater.h b/libs/hwui/DeferredLayerUpdater.h
index 61efafe..38d8e4a 100644
--- a/libs/hwui/DeferredLayerUpdater.h
+++ b/libs/hwui/DeferredLayerUpdater.h
@@ -68,7 +68,7 @@
 
     ANDROID_API void setTransform(const SkMatrix* matrix) {
         delete mTransform;
-        mTransform = matrix ? new SkMatrix(*matrix) : 0;
+        mTransform = matrix ? new SkMatrix(*matrix) : nullptr;
     }
 
     ANDROID_API void setPaint(const SkPaint* paint);