Add overrides and switch to nullptr keyword

Changes generated with clang-modernize.

Additionally, fixed some struct-vs-class usage to make clang happy.

Change-Id: Ic6ef2427401ff1e794d26f21f7b44868fc75fb72
diff --git a/libs/hwui/LayerCache.h b/libs/hwui/LayerCache.h
index 0ff8f3e..7d17b9b 100644
--- a/libs/hwui/LayerCache.h
+++ b/libs/hwui/LayerCache.h
@@ -96,10 +96,10 @@
 private:
     struct LayerEntry {
         LayerEntry():
-            mLayer(NULL), mWidth(0), mHeight(0) {
+            mLayer(nullptr), mWidth(0), mHeight(0) {
         }
 
-        LayerEntry(const uint32_t layerWidth, const uint32_t layerHeight): mLayer(NULL) {
+        LayerEntry(const uint32_t layerWidth, const uint32_t layerHeight): mLayer(nullptr) {
             mWidth = Layer::computeIdealWidth(layerWidth);
             mHeight = Layer::computeIdealHeight(layerHeight);
         }