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/RenderThread.h b/libs/hwui/renderthread/RenderThread.h
index 99c2e15..8fc8ca5 100644
--- a/libs/hwui/renderthread/RenderThread.h
+++ b/libs/hwui/renderthread/RenderThread.h
@@ -90,7 +90,7 @@
     EglManager& eglManager() { return *mEglManager; }
 
 protected:
-    virtual bool threadLoop();
+    virtual bool threadLoop() override;
 
 private:
     friend class Singleton<RenderThread>;