Default ringtone cannot be previewed as alarm stream type.

When choosing a ringtone for alarm use by "Android System"
ringtone picker (RingtonePickerActivity), default ringtone is
not played for preview with volume of alarm stream type but
with that of normal stream type.

It is because stream type is not reflected on mDefaultRingtone
instance of RingtonePickerActivity.

Change-Id: Idc71c9b620e40e4109604b2337677134bb74a8cd