update_engine: refactor into method am: 0dc25a6779

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2932913

Change-Id: Id5c123cbc026ed57ebc81d4379b81f4782c1ede4
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>