do not map partitions on A/B devices am: 50d4d9d80d

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/3476052

Change-Id: I11002f3029f1cf92d79fec3b4c58d812c600a36c
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>