Remove ICUService and decentralize GetAvailableLocales to each class.
Refactor part of the Intl::GetAvailableLocales into Intl::BuildLocaleSet
as helper function.

Bug: v8:5751
Cq-Include-Trybots: luci.v8.try:v8_linux_noi18n_rel_ng
Change-Id: Ic82d919cbf7ec840a7df3b0fa040561534c105a1
Reviewed-on: https://chromium-review.googlesource.com/c/1295934
Commit-Queue: Frank Tang <ftang@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56923}