ota_extractor: parallelize partition extractions am: c8c0057a7d

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/3164519

Change-Id: I3ba5f45ea75d0b1c799c8ecc99d31c631f1f3234
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>