NFC settings cleanup.

Remove NFC settings from Settings.System. The NFC Service tracks them
internally using SharedPreference.

Revert back to enable()/disable()/isEnabled() - Tag discovery is the
only functionality NFC service performs right now so we don't need to
break it up more explicitly yet. enable/disable is the global NFC
on/off and later we'll add enableDiscovery(), enableTagEmulation(),
enableSecureElement() etc as needed.

Change-Id: Id5b23e89d4a29511835f509962842452075e56cb
Signed-off-by: Nick Pelly <npelly@google.com>