Decode old fcmToken format

[paulb777]

Fix #12245.

If decoding of the FIRMessagingTokenInfo fails, fallback to the pre-10.19.0 code and migrate the keychain to the NSSecure encode.

Summary from @ncooke3:

1. developer onboards to 10.20.0+ → secure tokens are always read and written
2. developer updates to 10.20.0+ from 10.18.0 or below → initial read of an unsecure token, write back a secured token, and always read secure tokens afterwards
3. developer updates to 10.20.0+ from 10.19.0 → two subcases:
   a. the unsecure token was overwritten by the secure token & always read later as a secure token
   b. the unsecure token was NOT overwritten yet and the fallback decode code path kicks into effect

[google-oss-bot]

Size Report ¹

Affected Products

• FirebaseMessaging

  Type	Base (0fa8498)	Merge (5fb94a6)	Diff
  CocoaPods	?	-51.5 kB	? (?)

Test Logs

• Merge (5fb94a6): https://github.com/firebase/firebase-ios-sdk/actions/runs/7427300448

1. https://storage.googleapis.com/firebase-sdk-metric-reports/x3g0bwjjR6.html

[google-oss-bot]

Coverage Report ¹

Affected Products

• FirebaseMessaging-iOS-FirebaseMessaging.framework

  Overall coverage changed from ? (0fa8498) to 60.16% (5fb94a6) by ?.

  29 individual files with coverage change

  Filename	Base (0fa8498)	Merge (5fb94a6)	Diff
  FIRMessaging.m	?	55.20%	?
  FIRMessagingAnalytics.m	?	82.14%	?
  FIRMessagingAPNSInfo.m	?	87.76%	?
  FIRMessagingAuthKeychain.m	?	87.42%	?
  FIRMessagingAuthService.m	?	92.03%	?
  FIRMessagingBackupExcludedPlist.m	?	82.09%	?
  FIRMessagingCheckinPreferences.m	?	98.02%	?
  FIRMessagingCheckinService.m	?	86.49%	?
  FIRMessagingCheckinStore.m	?	81.68%	?
  FIRMessagingContextManagerService.m	?	85.71%	?
  FIRMessagingExtensionHelper.m	?	77.92%	?
  FIRMessagingKeychain.m	?	93.39%	?
  FIRMessagingLogger.m	?	100.00%	?
  FIRMessagingPendingTopicsList.m	?	89.02%	?
  FIRMessagingPersistentSyncMessage.m	?	38.10%	?
  FIRMessagingPubSub.m	?	52.14%	?
  FIRMessagingRemoteNotificationsProxy.m	?	76.35%	?
  FIRMessagingRmqManager.m	?	57.42%	?
  FIRMessagingSyncMessageManager.m	?	68.89%	?
  FIRMessagingTokenDeleteOperation.m	?	0.00%	?
  FIRMessagingTokenFetchOperation.m	?	75.91%	?
  FIRMessagingTokenInfo.m	?	66.92%	?
  FIRMessagingTokenManager.m	?	14.82%	?
  FIRMessagingTokenOperation.m	?	95.04%	?
  FIRMessagingTokenStore.m	?	53.17%	?
  FIRMessagingTopicOperation.m	?	0.00%	?
  FIRMessagingUtilities.m	?	56.73%	?
  NSDictionary+FIRMessaging.m	?	27.27%	?
  NSError+FIRMessaging.m	?	100.00%	?

Test Logs

• Merge (5fb94a6): https://github.com/firebase/firebase-ios-sdk/actions/runs/7427300448

1. https://storage.googleapis.com/firebase-sdk-metric-reports/YRK16Jxf2E.html

[ncooke3]

It would be better to fix the token in the keychain.

Agreed, but so far I can't think of such a way. My thinking is that in order to fix the token in the keychain, we'd have to read from it to know what it is, which leads back to the current decoding issue.