save subscription ID when upgrading from v3

We were not saving the OSUD_PUSH_SUBSCRIPTION_ID to user defaults when we detected a legacy player ID and proceed with upgrading.
It was only saved from create user responses that hydrate the subscription model, which does not happen on v3 -> v5 upgrade.

Author: nan-li

iOS_SDK/OneSignalSDK/OneSignalUser/Source/OneSignalUserManagerImpl.swift

@@ -228,6 +228,7 @@ public class OneSignalUserManagerImpl: NSObject, OneSignalUserManager {
         if let legacyPlayerId = OneSignalUserDefaults.initShared().getSavedString(forKey: OSUD_LEGACY_PLAYER_ID, defaultValue: nil) {
             OneSignalLog.onesignalLog(.LL_DEBUG, message: "OneSignalUserManager: creating user linked to legacy subscription \(legacyPlayerId)")
             createUserFromLegacyPlayer(legacyPlayerId)
+            OneSignalUserDefaults.initShared().saveString(forKey: OSUD_PUSH_SUBSCRIPTION_ID, withValue: legacyPlayerId)
             OneSignalUserDefaults.initStandard().removeValue(forKey: OSUD_LEGACY_PLAYER_ID)
             OneSignalUserDefaults.initShared().removeValue(forKey: OSUD_LEGACY_PLAYER_ID)
         } else {