Treat the default sound Uri the same as DEFAULT_SOUND.

For the purposes of deciding whether or not to invoke the
"fallback vibrate", that is. (RingtonePicker will return
content://settings/system/notification_sound when "Default"
is chosen, so if the app pops this Uri directly into
Notification.sound, we should treat it like DEFAULT_SOUND
and look to see whether the system notification sound is
not None before running the fallback vibration.)

Bug: 8627641
Change-Id: Ia469b8e4d5d7647ce1a8a179f591ed7a3443b5c5