Canonical IDs support

[ayushkr19]

Canonical IDs are sent as a response by the GCM server in case the registration ID changes for a client. The number of canonical IDs are given by the "canonical_ids" key in the response JSON. For the corresponding recipient, the registration_id on the server should be replaced by this canonical ID.

The response body format is given here: https://developers.google.com/cloud-messaging/http#response

Please check the 2nd example from this link : https://developers.google.com/cloud-messaging/http#example-responses