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

src/com/android/settings/datetime/timezone/BaseTimeZoneInfoPicker.java

@@ -23,6 +23,7 @@ import android.content.res.Resources;
 import android.icu.text.DateFormat;
 import android.icu.text.SimpleDateFormat;
 import android.icu.util.Calendar;
+import android.support.annotation.Nullable;
 
 import com.android.settings.R;
 import com.android.settings.datetime.timezone.model.TimeZoneData;
@@ -47,10 +48,17 @@ public abstract class BaseTimeZoneInfoPicker extends BaseTimeZonePicker {
     @Override
     protected BaseTimeZoneAdapter createAdapter(TimeZoneData timeZoneData) {
         mAdapter = new ZoneAdapter(getContext(), getAllTimeZoneInfos(timeZoneData),
-                this::onListItemClick, getLocale());
+                this::onListItemClick, getLocale(), getHeaderText());
         return mAdapter;
     }
 
+    /**
+     * @return the text shown in the header, or null to show no header.
+     */
+    protected @Nullable CharSequence getHeaderText() {
+        return null;
+    }
+
     private void onListItemClick(TimeZoneInfoItem item) {
         final TimeZoneInfo timeZoneInfo = item.mTimeZoneInfo;
         getActivity().setResult(Activity.RESULT_OK, prepareResultData(timeZoneInfo));
@@ -66,9 +74,11 @@ public abstract class BaseTimeZoneInfoPicker extends BaseTimeZonePicker {
     protected static class ZoneAdapter extends BaseTimeZoneAdapter<TimeZoneInfoItem> {
 
         public ZoneAdapter(Context context, List<TimeZoneInfo> timeZones,
-                OnListItemClickListener<TimeZoneInfoItem> onListItemClickListener, Locale locale) {
+                OnListItemClickListener<TimeZoneInfoItem> onListItemClickListener, Locale locale,
+                CharSequence headerText) {
             super(createTimeZoneInfoItems(context, timeZones, locale),
-                    onListItemClickListener, locale,  true /* showItemSummary */);
+                    onListItemClickListener, locale,  true /* showItemSummary */,
+                    headerText /* headerText */);
         }
 
         private static List<TimeZoneInfoItem> createTimeZoneInfoItems(Context context,