REF: EA value_counts -> _value_counts
Instead of returning a Series, return a tuple with the index and values to be passed to Series.

Where possible I've changed the methods to use `_values_for_factorized` in the hopes of converging on a base class implementation.  This is proving elusive, suggestions welcome.  cc @TomAugspurger @jorisvandenbossche 

xref #22843, #23074.