Reparent ChoiceCodecContext

Reparent ChoiceCodecContext

Choice as itself does not map to a DataObject, but rather to a ChoiceIn
-- it is the individual cases that also map to DataObject.

In this patch we reparent ChoiceCodecContext to be a subclass of
DataContainerCodecContext, which allows us to ditch the idea that a
there is a InstanceIdentifier.PathArgument corresponding to this
context.

A nice bonus is that we end up with exactly one serialization method,
which is delegating to the individual case.

Another nice thing is we have a natural place to host the case lookup
logic -- which fits squarely with
ChoiceCodecContext.bindingPathArgumentChild() contract.

JIRA: MDSAL-815
Change-Id: I12a80c849f6405e0e5723afc3f31704a2ad604a6
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>