Dynamically respond to notificationsV2 feature flag.

We were only reading it once, possibly before the flags were
initialized. This lets us be more responsive to the flag changing within
an application cycle.

app/src/main/java/org/thoughtcrime/securesms/dependencies/ApplicationDependencyProvider.java

@@ -186,14 +186,7 @@ public class ApplicationDependencyProvider implements ApplicationDependencies.Pr
 
   @Override
   public @NonNull MessageNotifier provideMessageNotifier() {
-    MessageNotifier inner;
-    if (FeatureFlags.useNewNotificationSystem()) {
-      inner = new MessageNotifierV2();
-    } else {
-      inner = new DefaultMessageNotifier();
-    }
-
-    return new OptimizedMessageNotifier(inner);
+    return new OptimizedMessageNotifier();
   }
 
   @Override