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/renderthread/CanvasContext.h b/libs/hwui/renderthread/CanvasContext.h
index d7f2ebd..d3fbde8 100644
--- a/libs/hwui/renderthread/CanvasContext.h
+++ b/libs/hwui/renderthread/CanvasContext.h
@@ -80,7 +80,7 @@
     void destroy();
 
     // IFrameCallback, Chroreographer-driven frame callback entry point
-    virtual void doFrame();
+    virtual void doFrame() override;
 
     void buildLayer(RenderNode* node);
     bool copyLayerInto(DeferredLayerUpdater* layer, SkBitmap* bitmap);