update_engine: factor out source_copy method am: 5a6e32b3b8

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2976857

Change-Id: I10daddd806fa9437ed1d53feb9feea8f98a073b0
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>