Simplify code that adds None as a valid timer ringtone

- None option is added to picker by default, remove code that
   manually added it
- Replace NO_RINGTONE_URI constant in TimerRingtonePreference
   with test method: DataModel.isSilentTimerRingtoneUri(uri)

Bug: 23499698
Change-Id: Id45a24bee607edd8295c2a31785e55d67b44e8b0