Fix settings crash when enhanced notification shown in search results

init NotificationBackend in constructor and set isSliceable to false in
search result since the NAS enabling logic is required and is in the
parent fragment ConfigureNotificationSettings.

Bug: 237251075
Test: test manually on device
Change-Id: I9082d6eda27784cf378a0d06304b5fc1e2ae6d7f
(cherry picked from commit 26ce9a98)