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/TreeInfo.h b/libs/hwui/TreeInfo.h
index e820b22..0799c6c 100644
--- a/libs/hwui/TreeInfo.h
+++ b/libs/hwui/TreeInfo.h
@@ -59,11 +59,11 @@
         : mode(mode)
         , prepareTextures(mode == MODE_FULL)
         , runAnimations(true)
-        , damageAccumulator(NULL)
+        , damageAccumulator(nullptr)
         , renderState(renderState)
-        , renderer(NULL)
-        , errorHandler(NULL)
-        , canvasContext(NULL)
+        , renderer(nullptr)
+        , errorHandler(nullptr)
+        , canvasContext(nullptr)
     {}
 
     explicit TreeInfo(TraversalMode mode, const TreeInfo& clone)