Use isGroupBlocked to check for blocked notifGroup

Avoids null pointer exception

Test: atest PreferencesHelperTest
Change-Id: Ic94b6f1f8768d4cb4afcdef8ec03165e6523188e
Fixes: 119798101
diff --git a/services/core/java/com/android/server/notification/PreferencesHelper.java b/services/core/java/com/android/server/notification/PreferencesHelper.java
index fd65ebe..eb46d53 100644
--- a/services/core/java/com/android/server/notification/PreferencesHelper.java
+++ b/services/core/java/com/android/server/notification/PreferencesHelper.java
@@ -1015,10 +1015,8 @@
 
     private boolean channelIsLive(PackagePreferences pkgPref, NotificationChannel channel) {
         // Channel is in a group that's blocked
-        if (!TextUtils.isEmpty(channel.getGroup())) {
-            if (pkgPref.groups.get(channel.getGroup()).isBlocked()) {
-                return false;
-            }
+        if (isGroupBlocked(pkgPref.pkg, pkgPref.uid, channel.getGroup())) {
+            return false;
         }
 
         // Channel is deleted or is blocked