Fix visibility and editability of importance fields

- Block field should always be visible
- Locked by OEM: cannot block or change importance
- Locked by default app: cannot block, can change importance
- Locked by system app: cannot block, can change importance
- system app but blockable: can block, can change importance

Test: robotests
Fixes: 131248127
Change-Id: Ifa718c84573dd5125aefa4f672a79dc4f267d515

src/com/android/settings/homepage/contextualcards/slices/NotificationChannelSlice.java

@@ -459,7 +459,7 @@ public class NotificationChannelSlice implements CustomSliceable {
     private boolean isChannelConfigurable(NotificationChannel channel,
             NotificationBackend.AppRow appRow) {
         if (channel != null && appRow != null) {
-            return !TextUtils.equals(channel.getId(), appRow.lockedChannelId);
+            return !channel.isImportanceLockedByOEM();
         }
 
         return false;