[lib] Handle messageStorePruneActionType in reduceThreadActivity

Summary:
We want to upsert lastPruned for threadIDs included in messageStorePruneActionType payload in threadActivityStore.

Depends on D9404

Test Plan:

1. Modify pruning code to run every minute
2. Observe that lastPruned inside threadActivityStore gets set as expected on messageStorePruneActionType

Reviewers: ashoat, ginsu, tomek

Reviewed By: ashoat

Subscribers: wyilio

Differential Revision: https://phab.comm.dev/D9406