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/ProgramCache.cpp b/libs/hwui/ProgramCache.cpp
index 6d3f0cb..3bbd520 100644
--- a/libs/hwui/ProgramCache.cpp
+++ b/libs/hwui/ProgramCache.cpp
@@ -429,7 +429,7 @@
     }
 
     auto iter = mCache.find(key);
-    Program* program = NULL;
+    Program* program = nullptr;
     if (iter == mCache.end()) {
         description.log("Could not find program");
         program = generateProgram(description, key);