update_engine: factor out source_copy method am: 5a6e32b3b8 am: 4b7655d03d Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2976857 Change-Id: If0269d04f1e45b5117db636ea9eeee521f38210c Signed-off-by: Automerger Merge Worker <[email protected]>