Don't corrupt parcel when writeFileDescriptor() fails

We now check for fd-legality before committing binder objects to
the flattened data buffer rather than after.  Previously we would
wind up corrupting the parcel and incurring driver-level errors,
as well as potentially leaking FDs.

Bug 21428802

Change-Id: Ice0d641b3dcc41fb1b8c68ce2e2ebd744c2863a1