update_engine: refactor manifest parsing am: 40354f8c56 am: cfa0ce8441

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

Change-Id: I1055294387a0d362442f7d9c48b5ec53792ecb73
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>