[MXKAccountManager] Handle UserNotifications framework API when checking local notifications settings

[SBiOSoftWhare]

-[MXKAccountManager isPushAvailable] should now handle iOS 10 UserNotifications framework to check if local notifications are enabled.

The problem is that with the UserNotifications framework the notification settings are available only asynchronously with [UNUserNotificationCenter getNotificationSettingsWithCompletionHandler:] method.

So isPushAvailable should evolve to be asynchronous now.

Note, it could also be great to:

• have a LocalNotificationService and RemoteNotificationService to decouple complexity
• avoid handling push notification from MXKAccount.