do not map partitions on A/B devices am: 50d4d9d80d am: 2583416115

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

Change-Id: If682ecd4a39a5b5a508965437fa24e4f38a2ca17
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>