Use token storage for tokens

[ahmedre]

This patch updates the library to use token storage for storage,
resulting in secure storage on both Android and iOS, instead of
SharedPreferences and NSUserDefaults. In iOS, it's stored in the
KeyChain, in Android, it's stored in DataStore but encrypted.

This also updates the normal storage to be stored in DataStore. On iOS,
this is a preference file in Application Support.

[ahmedre]

after these changes, initialization should look something like:

  // Android
  val graph = SharedDependencyGraph.init(
      driverFactory = DriverFactory(context = applicationContext),
      storage = createMobileSyncStorage(applicationContext),
      appEnvironment = AppEnvironment.PRELIVE,
      clientId = appClientId,
      clientSecret = appClientSecret
  )

  // iOS
  let storage = AppleMobileSyncStorageFactory.shared.create()

  graph = SharedDependencyGraph.shared.doInit(
      driverFactory: DriverFactory(),
      storage: storage,
      appEnvironment: AppEnvironment.prelive,
      clientId: appClientId,
      clientSecret: appClientSecret
  )