[REF] Update custom data handing in contact import

[colemanw]

Overview

Cleanup custom field handling in contact import.

Before

Over-reliance on htlm_type.

After

Uses more standard isSerialized method.

Comments

The other import classes got similar cleanup in #16979, which was merged by @Eileen. This one was a bit more cumbersome. I still consider the large amount of duplicated code across the 5 import classes to be a big FIXME, as it makes maintenance like this far more difficult than it should be.

The diff is not as large as it looks; changing the switch to if/else caused the indentation to change.

[civibot]

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[eileenmcnaughton]

The code makes sense & testing reveals bugs that are not made worse by this - ie importing custom fields of the following combos fails with an without this patch

Money->Select
Number->Text
Number->Select

leads to

I haven't dug further as my focus is to ensure no regression from this

[colemanw]

That bears further investigation. The Import classes are a mess; I'd like to clean up, consolidate and test their handling custom data, esp. in light of this movement to improve the custom field schema.