Always write SOURCE_COPY blocks in reverse order am: eb8703b9f6 am: fb6507adc0

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/1524238

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: I8a3a574a775870be39503cff41378b272fad22be