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

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

Change-Id: I74e430019f8b4fd4f2237226fac2c2d6831bead2
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>