libbinder_ndk: don't borrow fd into unique_fd

Some tests are checking that this fd isn't double-owned, but it appears
to be for a brief duration. Instead, avoiding creating a
ParcelFileDescriptor at all.

Fixes: 149851140
Test: CtsNdkBinderTestCases
Change-Id: Ibb90203bd7b9c9481756df64b84c048d01f0843e