Add Fd() method to file descriptor wrapper am: 32a1604c13

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2240835

Change-Id: I17afcecddd9c2208b85a43585d1ad72e0a5b88f2
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>