Refactor for ISnapshotWriter removal. am: a4b7ba6b50 am: c7144f0ef9

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2587387

Change-Id: I43a4f4815d7786620488ca8ad7938f2ebeebc3c5
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>