Revert "do not map partitions on A/B devices" am: 2f8976bd6a am: 36536fadae

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

Change-Id: Ic528158f02f5b8f818d980dd64edadb078428917
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>