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/Fence.h b/libs/hwui/Fence.h
index f175e98..fc29f7ac1 100644
--- a/libs/hwui/Fence.h
+++ b/libs/hwui/Fence.h
@@ -43,7 +43,7 @@
     Fence() {
         mDisplay = eglGetDisplay(EGL_DEFAULT_DISPLAY);
         if (mDisplay != EGL_NO_DISPLAY) {
-            mFence = eglCreateSyncKHR(mDisplay, EGL_SYNC_FENCE_KHR, NULL);
+            mFence = eglCreateSyncKHR(mDisplay, EGL_SYNC_FENCE_KHR, nullptr);
         } else {
             mFence = EGL_NO_SYNC_KHR;
         }