[native] Update nextMessagePruneTimeSelector to use threadActivityStore
Summary:
Update nextMessagePruneTimeSelector to use lastNavigatedTo/lastPruned from threadActivityStore instead of messageStore.threads.
IMPORTANT: Labeling as DRAFT because I'm not fully confident in the logic here yet. Putting this up to proceed with ripping out messageStore.threads.[lastNavigatedTo/lastPruned].
Depends on D9454
Test Plan: Change msInHour to small value to trigger pruning and log values. Verify pruning via Redux DevTools.
Reviewers: ashoat, tomek, ginsu, rohan
Reviewed By: ashoat
Subscribers: wyilio
Differential Revision: https://phab.comm.dev/D9469