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/accessibility/AccessibilitySettings.java

@@ -48,7 +48,6 @@ import com.android.settings.R;
 import com.android.settings.SettingsPreferenceFragment;
 import com.android.settings.Utils;
 import com.android.settings.search.BaseSearchIndexProvider;
-import com.android.settings.search.Indexable;
 import com.android.settingslib.RestrictedLockUtils;
 import com.android.settingslib.RestrictedLockUtils.EnforcedAdmin;
 import com.android.settingslib.RestrictedPreference;
@@ -75,7 +74,7 @@ import androidx.preference.SwitchPreference;
  */
 @SearchIndexable
 public class AccessibilitySettings extends SettingsPreferenceFragment implements
-        Preference.OnPreferenceChangeListener, Indexable {
+        Preference.OnPreferenceChangeListener {
 
     // Index of the first preference in a preference category.
     private static final int FIRST_PREFERENCE_IN_CATEGORY_INDEX = -1;