New manual time zone picker.

This implements a new manual time zone picker that allows selection of a
time zone for a selected country. It also allows selecting a fixed
offset time zone (most importantly Etc/UTC, which is a frequently
requested feature).

The new time zone picker is currently behind a feature flag
(settings_zone_picker_v2), which is disabled by default.

Test: manual
Test: SettingsFunctionalTests
Test: SettingsRobotTests
Bug: 62255208
Change-Id: I89c5a04bcb562b6facf5f31a8aa4ad1cdd51ab10