Notif redesign: Hide footer buttons when message is visible

When we show the text "Unlock to see older notifications", neither of
the settings/history buttons should be visible/tappable.

Test: FooterViewModelTest
Test: turn on feature to see only unseen notifs, lock device, check that
message looks good and no buttons are showing
Flag: com.android.systemui.notifications_redesign_footer_view
Bug: 375010573

Change-Id: I087ba490a42a42b18c7db9d72652ca1d6f3491d5