[binder] Switch on -Wzero-as-null-pointer-constant warning

libbinder and all its dependent libraries have been modenized to use
nullptr, switch on the warning to prevent new code from being checked in
that uses NULL macro.

Bug: 68236239
Test: m checkbuild
Change-Id: I0cac75884ac7d1e0ffa79f773157be37f0a40b6a
diff --git a/libs/binder/Android.bp b/libs/binder/Android.bp
index d642a48..4d95bf2 100644
--- a/libs/binder/Android.bp
+++ b/libs/binder/Android.bp
@@ -107,6 +107,7 @@
         "-Wall",
         "-Wextra",
         "-Werror",
+        "-Wzero-as-null-pointer-constant",
     ],
     product_variables: {
         binder32bit: {