Follow up from #1735118: Convert Field API to CMI
There's some overlap with #1959610: Remove public properties from entity classes I guess, but let's see.
The last submitted patch, field-api-protected-properties-1966008-3.patch, failed testing.
Right, let's wait for the outcome of #1959610: Remove public properties from entity classes (I have a feeling this is close to won't fix though)
New postponed one is #2016679: Expand Entity Type interfaces to provide methods, protect the properties
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