Fix custom ringtone restore in Notification channels.

Bug: 278169915
Test: atest PreferencesHelperTest && atest frameworks/base/core/tests/coretests/src/android/app/NotificationChannelTest.java
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:938eb0984d7672704ba3375d21b360010068f1c0)
Merged-In: I5c00b252054cf3736b3f24859a41d3fd894fe1de
Change-Id: I5c00b252054cf3736b3f24859a41d3fd894fe1de