Vertically centralize notifs on ls settings page entry point

Fix the issue that Notifications on lock screen text not
vertically centered in Lock screen page. Removed the
extra place-holder for summary. Also removed unused res in
lock_screen_notifications_settings.

Fix: 409386670
Flag: com.android.server.notification.notification_lock_screen_settings
Test: manual - Settings app - Display & touch - Lock screen
Change-Id: I8669e2e929c3749535085cc4ff4b607369bb4abc