Fix visibility of settings on channel page

Properly hide fields if the parent app or group
is blocked. This is needed because apps can link
directly to this screen.

Test: atest
Fixes: 130184191
Change-Id: I8c39410574940615fbb607a201e20e480ec02d87

tests/robotests/src/com/android/settings/notification/NotificationPreferenceControllerTest.java

@@ -117,6 +117,7 @@ public class NotificationPreferenceControllerTest {
     public void isAvailable_notIfChannelGroupBlocked() {
         NotificationBackend.AppRow appRow = new NotificationBackend.AppRow();
         NotificationChannel channel = mock(NotificationChannel.class);
+        when(channel.getImportance()).thenReturn(IMPORTANCE_DEFAULT);
         NotificationChannelGroup group = mock(NotificationChannelGroup.class);
 
         mController.onResume(appRow, channel, group, null);