Fix binder_to_string.h for NDK

AIDL NDK backend uses SpAIBinder and ScopedFileDescriptor for AIDL
IBinder and ParcelFileDescriptor types and they are handle types which
handles "null" cases by themselves.

Both types are specialized in ToString() so that they can print "(null)"
when managed resource(binder or fd) is null.

Bug: 181196266
Test: atest aidl_integration_test
Change-Id: I8fdd7ae2548b423d4cc4ffc56bf74521418de1d6