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/ShadowTessellator.cpp b/libs/hwui/ShadowTessellator.cpp
index 93d4b31..f8917e3 100644
--- a/libs/hwui/ShadowTessellator.cpp
+++ b/libs/hwui/ShadowTessellator.cpp
@@ -193,7 +193,7 @@
  * @param len the number of points of the polygon
  */
 bool ShadowTessellator::isClockwise(const Vector2* polygon, int len) {
-    if (len < 2 || polygon == NULL) {
+    if (len < 2 || polygon == nullptr) {
         return true;
     }
     double sum = 0;