Move some SOURCE_COPY op processing out of Init() function am: d11e2fcdbd

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2266600

Change-Id: Ic68dae2a911672c1cce3532fc7ab4bd27df6fe1a
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>