Change ranking update aconfig flag to use main bug

Test: aconfig build
Bug: 249848655
Change-Id: I0c359066800bd672c9a25440b7313e7217f1e481

core/java/android/service/notification/flags.aconfig

@@ -4,7 +4,7 @@ flag {
   name: "ranking_update_ashmem"
   namespace: "systemui"
   description: "This flag controls moving ranking update contents into ashmem"
-  bug: "284297289"
+  bug: "249848655"
 }
 
 flag {