IcuCollation: Sort digits under localised digits' headings

Previously both '1' and '۱' ("DIGIT ONE" and "EXTENDED ARABIC-INDIC
DIGIT ONE") were sorted under '1' heading, regardless of collation
locale.

Now they will be both sorted under localised heading name (transformed
using Language#formatNum), for example '1' for 'uca-en' collation or
'۱' for 'uca-fa' collation.

Bug: 55630
Change-Id: I0b745d955a6e72f53873c95648087aa5f90a8852