Fix partition mapping bug am: c675fc0aaa

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2134617

Change-Id: I4d2f1df51c6f1163bf6b6a6fa65d72987f6a12b9
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>