binder: fix FD handling in continueWrite am: e32c1ab25b am: d87b8c478c

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/native/+/29868448

Change-Id: Ib311677f7f594bb7f287312a96cfb0c355f7560a
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>