Add warning comment about serialization

As it's important we do not break serialization of KeyChainSnapshot
(as it could fail in weird and mysterious ways if we did), add
comments warning anybody editing those files to also update the
serializer and deserializer, as well as appropriate tests.

Test: none, just adding comments
Bug: 73921897
Change-Id: If73162b8fb2a0b44fd954b72c9030cd9e042282b