Add unlock to see more messaging to notif footer

If the device is locked, and there are no notifications on the keyguard,
and then one or more new notifications are received, and the user
expands the shade, if there would be additional notifications present if
the device is unlocked, then display a message in the footer.

Bug: 254647461
Test: manual
      1: Enable "Only Show Unseen Notifs On Keyguard" flag
      2: Have some notifications
      3: Lock device
      4: On keyguard, observe no notifications
      5. Receive a notification.
      6: Expand shade (without unlocking)
      Observe: New UI treatment is visible
Change-Id: Ibe4c3f955229ab78845a6f51adffc30ab355f52f