Add a header view to show the country in RegionZonePicker

Extra fixes in this CL
- Minor string update in time zone picker.
  Use date_time_search_region string in search bar,
  and date_time_set_timezone_title string for lower case "zone".
- Fixed b/76893139. Remove the unnecessary top padding in RecyclerView.
  Create a new layout file time_zone_items_list.xml without the padding.
- Add missing return statement when region ISO code
  is invalid in RegionZonePicker#getAllTimeZoneInfos

Bug: 76209571
Bug: 76893139
Test: m RunSettingsRoboTests ROBOTEST_FILTER=com.android.settings.datetime.timezone
Test: Verified that the strings are updated in the UI
Change-Id: I1fb3e2abf80da3cb53cfbc3363bbe46e40a6ac22