Push notification server key

If we use gcm key that starting https://android.googleapis.com/gcm/send instead of fcm one, we must use Legacy server key of firebase console and matched sender_id of manifest.json. If we don't gcm server will return sender mismatch error like below { multicast_id: 7084886970116218000, success: 0, failure: 1, canonical_ids: 0, results: [ { error: 'MismatchSenderId' } ] }