Revert "do not map partitions on A/B devices" am: 2f8976bd6a

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/3480990

Change-Id: Iccbb89c5d52879106ad4e76687a4fc17b229a05c
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>