ota_extractor: parallelize partition extractions am: c8c0057a7d am: b8d305be84

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

Change-Id: I8043d436d4eb46c094a78c356c4d1d4010f3b2c1
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>