Free ISnapshotManager once update is done am: 47061dfdc4

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/3506416

Change-Id: If1693817a9976f8e8a7b9b83fbc68bb706784df1
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>