Add UI tests for new time zone picker

- Test selecting 2 time zones in picker
- Device.wait could return null object and cause NPE.
  Add helper function throw UiObjectNotFoundException with selector info

Bug: 62255208
Test: atest SettingsUITests:ZonePickerSettingsTest
Change-Id: Id0439dde25240cf2d3ee834043d1726a2c738d1a