Settings: Fix NPE in ChannelNotificationSettings

java.lang.NullPointerException: Attempt to invoke virtual method 'int
android.app.NotificationChannel.getImportance()' on a null object
reference at com.android.settings.notification.ChannelNotificationSettings.
onActivityResult.

Change-Id: I3ec7b51e2533dc108b1a14c151e3ad70d22113e5
CRs-Fixed: 2068192
Fixes: 64116799
Fixes: 64134707
Test: monkey