[nit] display event does not need preventDefault state

* It can just use the `wantsToDisplay` property of the notification it owns.

Author: nan-li

iOS_SDK/OneSignalSDK/OneSignalNotifications/OSNotification+OneSignal.m

@@ -85,6 +85,10 @@ - (void)complete:(OSDisplayableNotification *)notification {
     }
 }
 
+- (BOOL)wantsToDisplay {
+    return _wantsToDisplay;
+}
+
 - (void)setWantsToDisplay:(BOOL)display {
     _wantsToDisplay = display;
 }

iOS_SDK/OneSignalSDK/OneSignalNotifications/OSNotificationsManager.m

@@ -93,13 +93,12 @@ @interface OSDisplayableNotification ()
 - (void)startTimeoutTimer;
 - (void)setCompletionBlock:(OSNotificationDisplayResponse)completion;
 - (void)complete:(OSDisplayableNotification *)notification;
+- (BOOL)wantsToDisplay;
 - (void)setWantsToDisplay:(BOOL)display;
 @end
 
 @implementation OSNotificationWillDisplayEvent
 
-BOOL _isPreventDefault = false;
-
 - (id)initWithDisplayableNotification:(OSDisplayableNotification*)notification {
     self = [super init];
     if(self) {
@@ -109,12 +108,11 @@ - (id)initWithDisplayableNotification:(OSDisplayableNotification*)notification {
 }
 
 - (BOOL)isPreventDefault {
-    return _isPreventDefault;
+    return !_notification.wantsToDisplay;
 }
 
 - (void)preventDefault {
     [OneSignalLog onesignalLog:ONE_S_LL_VERBOSE message:[NSString stringWithFormat:@"OSNotificationWillDisplayEvent.preventDefault called."]];
-    _isPreventDefault = true;
     _notification.wantsToDisplay = false;
 }