update_engine: refactor manifest parsing am: 40354f8c56

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2938200

Change-Id: If0bec7dc819975b56064fd4b3f979fda286679c3
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>