Move serializ. logic into EntityContentDataCodec

This moves the usage of the recently introduced Serializer/
Deserializer classes into EntityContentDataCodec, reducing the
need for injection into EntityHandlers.

This prepares the current code base for the introduction of
redirects.

Change-Id: I285c6b496b27971198cf2b30aeccf19be28b8bd8
Merge "Move serializ. logic into EntityContentDataCodec"