Reduce number of partition map calls am: 291ac8c423

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/3277112

Change-Id: I23f4ab1962fd5a245e7206b620de0203ea998904
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>