Increase the number of notification items that can be shown in Notification...

Increase the number of notification items that can be shown in Notification history - Recently dismissed

 Increase the max number of items shown in 'Notification history - Recently dismissed' from 6 to 50.

Bug: 255784319
Test: Manual: Dismiss 6 or more notifications, open Settings-Notifications-Notification History, the max item count under `Recently dismissed` should be 50.
Change-Id: Ia13158ae1ebca448d43bf4c6f6037758b3f40615