Allow including file_descriptor.h in fs_mgr. am: 5e6dfe3933 am: e44bd2ef37 am: d3338f3b86

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/1430218

Change-Id: I71fe539429d2241cf33e37b868c3ae19357f8beb