Limit partition parallelization to number of CPU cores am: d021d81cc5

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/3309176

Change-Id: I5e85800541a75350e746e72d09e543f86d1056fb
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>