Misc tweaks to Indexable usage and tests.

- Make SettingsPreferenceFragment implement Indexable directly
- Add existing fragments that are SettingsPrefFragment but not Indexable
  to grandfather list.
- Fix "font size" fragment so it shows up in search properly.
- Remove duplicates for "lock screen display", "Screen saver".

Bug: 70720645
Test: robotests
Change-Id: I44ce0cac0fdf5e882b315861bb4bebc45dbe01e6

src/com/android/settings/display/NightDisplaySettings.java

@@ -42,7 +42,7 @@ import androidx.preference.Preference;
  */
 @SearchIndexable(forTarget = SearchIndexable.ALL & ~SearchIndexable.ARC)
 public class NightDisplaySettings extends DashboardFragment
-        implements ColorDisplayController.Callback, Indexable {
+        implements ColorDisplayController.Callback {
 
     private static final String TAG = "NightDisplaySettings";