sounds: Use silent if URI returned by ringtone picker is null

Based on RingtoneManager's documentation [1], the EXTRA_RINGTONE_PICKED_URI
will be null if the "Silent" item was picked.

[1]: https://developer.android.com/reference/kotlin/android/media/RingtoneManager#EXTRA_RINGTONE_PICKED_URI:kotlin.String

Change-Id: I5898d64d898d0762c3a8836848f956d876a27d5c