Fix Conversation widget on new notif pipeline

The Conversation widget stopped updating properly because it was
updating on a background thread but it needed to get all active and
pending notifications to do that, and the new pipeline (but not the old)
only allows that on the main thread.

This change gets the notifications on the main thread, then passes the
list to the background thread to finish the task.

Bug: 225795397
Test: manual
Change-Id: If05edbfd1817e5e28dc5a18c892fdb52ef1075c1