Show time zone used in or after 2018 only

- Year 2018 is picked as the new time zone data
for picker is introduced in 2018. I can be adjusted
in the future to show more merged time zones.
- Add test to ensure each region has at least
one time zone.
- Fix a issue not using a singleton of TimeZoneData
  in commit 6c33caad4b

Bug: 72142943
Test: atest SettingsUnitTests:TimeZoneDataTest
Change-Id: I5eba7fbd59a2d3fd2e8062c9615946dc4bbb6314

src/com/android/settings/datetime/timezone/model/TimeZoneDataLoader.java

@@ -32,7 +32,7 @@ public class TimeZoneDataLoader extends AsyncLoader<TimeZoneData> {
     @Override
     public TimeZoneData loadInBackground() {
         // Heavy operation due to reading the underlying file
-        return new TimeZoneData();
+        return TimeZoneData.getInstance();
     }
 
     @Override