Reduce number of partition map calls am: 291ac8c423 am: a6b74157e5

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

Change-Id: I029ad9a0534a4f4fc1f71ed181f3ac719325ebf7
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>