Modernize codebase by replacing NULL with nullptr

Fixes -Wzero-as-null-pointer-constant warning for binder.

Test: m
Bug: 68236239
Change-Id: I8184bd6aa4ebff1bd8c88dad16886e98df853b03

libutils/include/utils/Singleton.h

diff --git a/libutils/include/utils/Singleton.h b/libutils/include/utils/Singleton.h
index 2dd5a47..44d8ad7 100644
--- a/libutils/include/utils/Singleton.h
+++ b/libutils/include/utils/Singleton.h
@@ -51,7 +51,7 @@
     static TYPE& getInstance() {
         Mutex::Autolock _l(sLock);
         TYPE* instance = sInstance;
-        if (instance == 0) {
+        if (instance == nullptr) {
             instance = new TYPE();
             sInstance = instance;
         }
@@ -60,7 +60,7 @@
 
     static bool hasInstance() {
         Mutex::Autolock _l(sLock);
-        return sInstance != 0;
+        return sInstance != nullptr;
     }
     
 protected:
@@ -90,7 +90,7 @@
 #define ANDROID_SINGLETON_STATIC_INSTANCE(TYPE)                 \
     template<> ::android::Mutex  \
         (::android::Singleton< TYPE >::sLock)(::android::Mutex::PRIVATE);  \
-    template<> TYPE* ::android::Singleton< TYPE >::sInstance(0);  /* NOLINT */ \
+    template<> TYPE* ::android::Singleton< TYPE >::sInstance(nullptr);  /* NOLINT */ \
     template class ::android::Singleton< TYPE >;