Toggle to switch recent panels style (2/2)

- Allow us to switch between Stock or Slim recents panel;
- Properly grey options in recents panel settings area

This implementation was based in: https://github.com/AOSB/android_packages_apps_Settings/commit/cc8853be9f7df9425991ace049f7b109178007b3
Thanks @codex-corp

Change-Id: Iba1f9ccc396db85cd08c690b82f7717819271a99