Limit partition parallelization to number of CPU cores am: d021d81cc5 am: c3bc14d3e5

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

Change-Id: I9b384a99c039869de5ef32697b6464935c32a80a
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>