[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
1 file changed