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/Patch.cpp b/libs/hwui/Patch.cpp
index 442dc5b..8a4d65b 100644
--- a/libs/hwui/Patch.cpp
+++ b/libs/hwui/Patch.cpp
@@ -71,7 +71,7 @@
     uint32_t yCount = patch->numYDivs;
 
     uint32_t maxVertices = ((xCount + 1) * (yCount + 1) - emptyQuads) * 4;
-    if (maxVertices == 0) return NULL;
+    if (maxVertices == 0) return nullptr;
 
     vertices.reset(new TextureVertex[maxVertices]);
     TextureVertex* vertex = vertices.get();