Place call to completionHandler outside rescinding callback so that we avoid app being killed by the system

Summary: This diff moves call to completionHandler from rescinding callback in AppDelegate in handleBackgroundNotification. As established before rescinding callback is highly unlikely to ever be called, but failure to call completionHandler will result in app being terminated by the system

Test Plan: Launch the app from XCode and put it into background mode. Open two web clients one of which is the same as mobile one. Send messages and read them on web client several times waiting a couple of minutes between each trial. Ensure that app is still running.

Reviewers: palys-swm, atul, ashoat

Subscribers: ashoat, Adrian, yayabosh

Differential Revision: https://phab.comm.dev/D4573