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