[quick_action] fix delegate methods not called on iOS

packages/quick_actions/ios/Classes/FLTQuickActionsPlugin.m

@@ -19,6 +19,7 @@ + (void)registerWithRegistrar:(NSObject<FlutterPluginRegistrar> *)registrar {
   FLTQuickActionsPlugin *instance = [[FLTQuickActionsPlugin alloc] init];
   instance.channel = channel;
   [registrar addMethodCallDelegate:instance channel:channel];
+  [registrar addApplicationDelegate:instance];
 }
 
 - (void)handleMethodCall:(FlutterMethodCall *)call result:(FlutterResult)result {