Canonicalize notification channel sounds for backup

Canonicalize for backup and canonicalize and uncanonicalize for restore
(see comment).

Test: Set custom notification sound, make backup, remove notification
sound from device (from Ringtones and make sure to update media content
provider), restore => Observe default instead of random number. Do the
same without removing the sound and observe restores successfully.
Test: runtest systemui-notification
Bug: 66444697
Change-Id: Id2f4685b7f64a35e397eb6dff5246802d795d996