[native] [ENG-2207] Too big notifications when the message is long

Summary:
Long messages lead to in-app notifications taking up almost half the screen on iOS and Android.

Test Plan:
I tested new behavior on both operating systems. Now it shows only the first line of the message.

Reviewers: tomek

Reviewed By: tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6685