[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

lib/reducers/thread-activity-reducer.js

@@ -1,5 +1,6 @@
 // @flow
 
+import { messageStorePruneActionType } from '../actions/message-actions.js';
 import {
   deleteThreadActionTypes,
   leaveThreadActionTypes,
@@ -29,6 +30,19 @@ function reduceThreadActivity(
       },
     };
     return updatedThreadActivityStore;
+  } else if (action.type === messageStorePruneActionType) {
+    const now = Date.now();
+    let updatedThreadActivityStore = { ...state };
+    for (const threadID: string of action.payload.threadIDs) {
+      updatedThreadActivityStore = {
+        ...updatedThreadActivityStore,
+        [threadID]: {
+          ...updatedThreadActivityStore[threadID],
+          lastPruned: now,
+        },
+      };
+    }
+    return updatedThreadActivityStore;
   } else if (
     action.type === leaveThreadActionTypes.success ||
     action.type === deleteThreadActionTypes.success