Save push subscription ID on app start

Because of a bug prior to v5.0.5, players that upgraded from v3 -> v5 were missing the player ID in cache for sending to notification opened and notification received endpoints.
Now, every time app cold starts, as part of the push subscription model checking itself for updates, we will cache its subscription ID if non-empty.

Author: nan-li

iOS_SDK/OneSignalSDK/OneSignalUser/Source/OSSubscriptionModel.swift

@@ -425,6 +425,9 @@ extension OSSubscriptionModel {
         netType = OSNetworkingUtils.getNetType() as? Int
         // sdkType ??
         // isRooted ??
+        if type == .push && !(subscriptionId ?? "").isEmpty {
+            OneSignalUserDefaults.initShared().saveString(forKey: OSUD_PUSH_SUBSCRIPTION_ID, withValue: subscriptionId)
+        }
     }
 
     enum OSPushPropertyChanged {