NdkBinderTest: sane defaults for null args

These were carefully crafted, and they are unlikely to be needed to
change at this point. Solidifying them in the test because this is the
kind of things apps are likely to rely on.

Bug: 111445392
Test: atest android.binder.cts
Change-Id: I1e43e66dca62d4ce6861875c8ff07bb7df4d01ae