Rewrite pref cleanup script

- Keep the current hidden pref cleanup stuff, that's not harmful
  and marginally useful
- Drop preferences we dunno wtf they're about. Cuz they're probably
  deprecated or otherwise unused
- Normalize preferences into accepted value ranges. This part is kinda
  hard and I haven't figured it out, so slap a TODO

More to come, stay tuned!

Change-Id: I70047adba0034136d107ce7534294cc6fa3c1860