libbinder_ndk: make test work w/ TEST_MAPPING

Since it was broken.

This was two modules, but it's better to make it one (and fork w/
PR_SET_PDEATHSIG so that the test doesn't get into a bad state).

Test: libbinder_ndk_unit_test
Change-Id: I928fe70b401f7c523f664c898f882afe08aec741