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/Snapshot.cpp b/libs/hwui/Snapshot.cpp
index cf8229fd..d4ca99e 100644
--- a/libs/hwui/Snapshot.cpp
+++ b/libs/hwui/Snapshot.cpp
@@ -29,16 +29,16 @@
 
 Snapshot::Snapshot()
         : flags(0)
-        , previous(NULL)
-        , layer(NULL)
+        , previous(nullptr)
+        , layer(nullptr)
         , fbo(0)
         , invisible(false)
         , empty(false)
         , alpha(1.0f)
-        , roundRectClipState(NULL) {
+        , roundRectClipState(nullptr) {
     transform = &mTransformRoot;
     clipRect = &mClipRectRoot;
-    region = NULL;
+    region = nullptr;
     clipRegion = &mClipRegionRoot;
 }
 
@@ -80,7 +80,7 @@
         flags |= Snapshot::kFlagFboTarget;
         region = s->region;
     } else {
-        region = NULL;
+        region = nullptr;
     }
 }