Fix UX problems in time zone pickers

- Remove emoji region flag in the region picker.
  It's more consistent with locale picker which shows no flag in region
  picker
- Remove redundant information in the summary field
  e.g. same GMT offset in primary and secondary field in fixed offset
  picker
- Better mode switching flow. Switch region/fixed offset mode
only when the user confirms their selection in the picker.

Bug: 73952488
Test: m RunSettingsRoboTests ROBOTEST_FILTER=com.android.settings.datetime.timezone
Change-Id: Id5da8a2516acd10c9a3d71181e94bc617d797d20