Don't unmap partitions if they weren't mapped am: 24287c3fb1 am: fee32fddbb

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2480296

Change-Id: I6685d557c5d81ea2f342d50b4779622cd7d51d11
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>