[lib] Make lastPruned and lastNavigatedTo type optional for thread activity entry

Summary:
This makes the lastPruned or lastNavigatedTo type field optional for thread activity entries (both empty is not possible). Currently, we populate thread activity entries lazily, and so the type should reflect that either field is optional.

Context: https://linear.app/comm/issue/ENG-7663/missing-lastpruned-field-in-current-thread-activity-redux-persist#comment-d17294bb

Test Plan: flow check

Reviewers: atul

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11622