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/Dither.cpp b/libs/hwui/Dither.cpp
index 77006a4..12d9389 100644
--- a/libs/hwui/Dither.cpp
+++ b/libs/hwui/Dither.cpp
@@ -24,7 +24,7 @@
 // Lifecycle
 ///////////////////////////////////////////////////////////////////////////////
 
-Dither::Dither(): mCaches(NULL), mInitialized(false), mDitherTexture(0) {
+Dither::Dither(): mCaches(nullptr), mInitialized(false), mDitherTexture(0) {
 }
 
 void Dither::bindDitherTexture() {