RingtoneManager: Don't crash if the ringtone doesn't have a numerical ID

When changing the notification sound for some apps with app-provided sounds,
the notification URI doesn't have a numerical ID.
For example, Outlook's email notification URI has the id 'new_email'

Test: m, outlook doesn't crash anymore, test google calendar

Change-Id: Ibf0c6f2f13e1c76ce02784165b97b92e8f7e432c