libbinder_ndk: call markForBinder internally

To support using libbinder_ndk w/ sockets.

This doesn't make a lot of sense without NDK APIs to get ahold of
binders, but platform utilities like the test here don't need those.

Bug: 167966510
Test: binderRpcTest
Change-Id: I63739a0d971e05b71908140caf6c6221fde48564