remember
Builds the CountryListIndex for sections, recomputed only when the sections change.
Letters are taken from the unsectioned group only (CountrySectionKind.All). Indexing the "Recently selected" or "Suggested" groups too would make one letter map to several places and the rail jump backwards — those groups are short, deliberately at the top, and reachable by scrolling.
The initial letter is read from the accent-folded name, so Åland indexes under A rather than after Z where a raw comparison puts it.