Fix for UnmapPartitionOnDeviceMapper am: 94fa3e525f

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/3380291

Change-Id: Ideb976cb7bf46c4d1723bb4ebafce83722504ae6
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>