Push notifications for Gmail to trigger updates

[DenBond7]

          After some research and testing, I have the following result:

1. To be able to receive faster updates about messages(new, update, and so on) on Android we have to use push notifications. It's only one way for now to work effectively. All other cases tasted on my side or have bad(unsafe) approaches or don't work well due to different reasons(it depends on a particular device or Android behavior at all)
2. @martgil So for now we have the same situation: on Android, updates can be fetched once per 15 minutes or less automatically. If a user need to have the latest info he has to do pull-to-refresh manually.
3. @sosnovsky I don't know how you are going to integrate real-time notifications about updates on iOS. If you can use Firebase Cloud Messaging maybe we can have only one backend solution for both platforms. For now, without a backend solution, I can't show updates faster. @sosnovsky @tomholub Please let me know if we are interested in it and I will create a new issue where we can discuss it.

Originally posted by @DenBond7 in #2141 (comment)