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/DamageAccumulator.cpp b/libs/hwui/DamageAccumulator.cpp
index 420e331..a422622 100644
--- a/libs/hwui/DamageAccumulator.cpp
+++ b/libs/hwui/DamageAccumulator.cpp
@@ -79,7 +79,7 @@
 void DamageAccumulator::pushCommon() {
     if (!mHead->next) {
         DirtyStack* nextFrame = (DirtyStack*) mAllocator.alloc(sizeof(DirtyStack));
-        nextFrame->next = 0;
+        nextFrame->next = nullptr;
         nextFrame->prev = mHead;
         mHead->next = nextFrame;
     }
@@ -147,7 +147,7 @@
             return frame;
         }
     }
-    return NULL;
+    return nullptr;
 }
 
 static DirtyStack* findProjectionReceiver(DirtyStack* frame) {
@@ -160,7 +160,7 @@
             }
         }
     }
-    return NULL;
+    return nullptr;
 }
 
 static void applyTransforms(DirtyStack* frame, DirtyStack* end) {