Fix BINDER_LIB_TEST_NOP_CALL_BACK.

To write in the request Parcel instead of the reply Parcel.

Also fixed reading the callback return value to use
readInt32(&...), so we correctly set the status if
there was no data in the Parcel.

Test: binderLibTest passes
Change-Id: I1c5b075b6f1f6d19c40e7bf9589a94809ce414ff