libbinder_ndk unit tests: hold onto linked binder

It's now required to hold onto a binder if you are linked to death to
it, so that there is no memory leak when someone forgets to unlink. This
is consistent with the libbinder_ndk documentation.

Bug: 145692765
Test: ./runtests.sh
Change-Id: Id7622301ed2691ae2e4bfa472a332e3823e70003