Refactor for ISnapshotWriter removal. am: a4b7ba6b50

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

Change-Id: I73eb31487ab5851e65226e5d6f24dfee0e5773ee
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>