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

(cherry picked from commit c27bb6ad)

Change-Id: I32c186d0d7479b01f6cc67cce9bc5cb66264a064