ota_extractor: functionize sub-partition handling am: 202dee2281

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

Change-Id: I2474c726794fce55cc4c0745e2617f864e32103a
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>