Introducing EntityContentDataCodec

This factors the basic serialization logic for EntityContent
into EntityContentDataCodec, removing any knowledge about the
serialization from EntityHandler and EntityFactory.

Bug: 66024
Change-Id: I90d38bda79aa10f2292432dda28dd97453d47321
Introducing EntityContentDiff

EntityContentDiff handles redirects if present,
and uses EntityDiff otherwise.

Bug: 66026
Change-Id: I0d8798cf18bf8375af9813d446862f5d56fb1b66
Merge "Introducing EntityContentDataCodec"
Merge "Introducing EntityContentDiff"