[REF] CustomField code cleanup to use isSerialized method

[colemanw]

Overview

Part of a series of code refactors to reduce guesswork about custom field serialization.

Before

Relies on html_type to guess at serialization

After

Uses isSerialized

[civibot]

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[pradpnayak]

Changes look good, will do some test soon.

[pradpnayak]

Tested by creating and updating fields through UI and api. Data against record is saved and retrieved correctly against the entity.

Good to merge.