SystemUI: show LTE signal strength in 5G NSA

UI shows 5G icon with empty signal bar when NR modem is not in ENDC
connected mode. It looks a bit misleading to show empty signal bar

To provider a better user experience, show LTE signal strength in
5G NSA when 5G signal strength is invalid

Change-Id: I0fc4122605adb5782a5853d7c0d61c9fef6150e1
CRs-Fixed: 2488906