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

tests/robotests/src/com/android/settings/display/FontSizePreferenceControllerTest.java

@@ -49,7 +49,7 @@ public class FontSizePreferenceControllerTest {
     @Test
     public void isAlwaysAvailable() {
         assertThat(mController.getAvailabilityStatus())
-                .isEqualTo(BasePreferenceController.AVAILABLE);
+                .isEqualTo(BasePreferenceController.AVAILABLE_UNSEARCHABLE);
     }
 
     @Test