Alarms created from external intents use default ringtone

Clock recently gained the notion of "application default ringtone"
but this was ignored in favor of "system default alarm ringtone" when
creating alarms from external intents. This behavior was fixed.

A secondary cleanup moved the functionality in
RingtoneDataManager into DataModel.

Bug: 23353723
Change-Id: Id092101dc30ec6c48f59cf4f142778b08a6075de