ota_extractor: functionize sub-partition handling am: 202dee2281 am: 03b600f431

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/3164518

Change-Id: Ifd5615d7ccc5be137fb6cb6d930d08190c662297
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>