Setting to control seen notification filtering

Test: manual
  1. adb shell settings put secure \
     lock_screen_show_only_unseen_notifications <0|1>
  2. Settings > Notifications > General > Show only new notifications on
     lock screen
  3. Toggle setting
  Observe: if setting is enabled, seen notifications are filtered from
           the lockscreen, and vice versa
Bug: 254647461
Change-Id: I4f6e35a1d918095cea25a97f72ddd08869ad9b31