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/SpotShadow.cpp b/libs/hwui/SpotShadow.cpp
index b3cfde2..a03192a 100644
--- a/libs/hwui/SpotShadow.cpp
+++ b/libs/hwui/SpotShadow.cpp
@@ -332,7 +332,7 @@
  * @param len the number of points of the polygon
  */
 void SpotShadow::makeClockwise(Vector2* polygon, int len) {
-    if (polygon == 0  || len == 0) {
+    if (polygon == nullptr  || len == 0) {
         return;
     }
     if (!ShadowTessellator::isClockwise(polygon, len)) {