Send GraphQlProvider.chatItems() in handlePushNotification()

[SleepySquash]

Synopsis

Message should be considered delivered as soon as FCM notification is received.

Solution

This PR adds the query of GraphQlProvider.chatItems() when a FCM notification is received, so that it's received status is updated.

Checklist

• Created PR:
  • In draft mode
  • Name contains issue reference
  • Has type and k:: labels applied
• Before review:
  • Documentation is updated (if required)
  • Tests are updated (if required)
  • Changes conform code style
  • CHANGELOG entry is added (if required)
  • FCM (final commit message) is posted or updated
  • Draft mode is removed
• Review is completed and changes are approved
  • FCM (final commit message) is approved
• Before merge:
  • Milestone is set
  • PR's name and description are correct and up-to-date
  • All temporary labels are removed

[SleepySquash]

Let's summarize what's been discovered:

1. Android allows push notifications to run its background handler any time and in any condition. Thus sending a query to mark some ChatItems as delivered is possible.
2. iOS won't work: background handler is invoked for background notifications only (visible ones won't call that). Also when application is terminated, it seems like the handler isn't invoked at all. Perhaps it can be called only once in 15 minutes?

[SleepySquash]

Moved to #1038