Send deviceToken to Ably clients which already have been instantiated

[ben-xD]

This PR does 2 things:

• It fixes Investigate push activation not completing on iOS when running in release configuration #276
• It allows users to give Ably flutter the device token if they're having issues activating push with Ably. This happens because in some cases (e.g. a customer), didRegisterForRemoteNotifications does not get called in Ably Flutter when running on iOS. This might happen due to method swizzling by other dependencies. This is a workaround for users who cannot fix the fact didRegisterForRemoteNotifications is not being called in Ably Flutter.

[ben-xD]

@lukasz-szyszkowski , I would like to change the pattern of calling a method in the singleton instance of AblyInstanceStore, seen in AblyInstanceStore.sharedInstance().didRegisterForRemoteNotifications(withDeviceToken: deviceToken), to a static method in AblyFlutter itself:

For example, it would be nicer if the user did:

AblyFlutter.didRegisterForRemoteNotifications(withDeviceToken: deviceToken)

Could I get some help with improving this code?

[ikbalkaya]

LGTM

[QuintinWillison]

We're pressured for time to get a release out. So, I'm going to land this work despite having not observed checks pass yet, due to the fact that the most recent commit should not fail CI and the preceding commit passed CI.