libsnapshot: Restrict when snapshots can be deleted.

Do not delete snapshots for any reason other than merge-complete or a
cancel request from update_engine. Additionally, create a rollback
indicator file if booting into the source slot, so update_engine can
attempt to cancel the update.

Bug: 147819418
Bug: 147347110
Test: libsnapshot_test gtest
Change-Id: Id357a91cec467a60246c7c3d133f6c54ccb3fc93