Update pruneThreadIDsSelector to use lastNavigatedTo from threadActivityStore instead of messageStore.threads.

Also as discussed w/ @ashoat, we can check threadMessageInfo.messageIDs.length instead of lastPruned to reduce unnecessarily adding threadIDs to threadIDsToPrune. lastPruned will be removed from threadActivityStore in a subsequent diff.

Depends on D9453

Change msInHour to small value to "trigger" pruning and log threadIDsToPrune. Verify pruning via Redux Devtools. Have done minimal preliminary testing, will test more thoroughly before landing.