Implement activation/ deactivation/ updating lifecycle method

[ben-xD]

We need to implement a way of checking when activate/ deactivate succeeds/ fails and to find out when an update with Ably fail. In the current PR #140, users won't know if activation succeeds or fails.

In iOS, ARTPushRegistererDelegate is the delegate with 3 methods which are called back:

• didActivateAblyPush: called when activate succeeds or fails
• didDeactivateAblyPush: called when deactivate succeeds or fails
• didAblyPushRegistrationFail: called when updating Ably with new details fails. The point is, it is only called when updating ably (in an internal method called callUpdateFailedCallback), and not during "registration"/ activation.

In Android, there are 3 intents which are created. The user has to listen to these:

• io.ably.broadcast.PUSH_ACTIVATE
• io.ably.broadcast.PUSH_DEACTIVATE
• io.ably.broadcast.PUSH_UPDATE_FAILED

┆Issue is synchronized with this Jira Task by Unito

[ben-xD]

This was added in #140, and is now documented in https://github.com/ably/ably-flutter/blob/main/PushNotifications.md#device-activation