[lib] fix PushHandler to support DMs

Summary: Solution described in ENG-9465.

Test Plan:
Apply this diff:

1. Comment this line to make sure we're testing actual notifs code & add some logs to make sure code in reducer was executed.
2. Open physical device
3. In DM thread send a test message
4. Notif delivered
5. Message immediately visible in UI
6. Thread marked as unread
7. Logs from reducer are there

Reviewers: ashoat, kamil

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D13576