Do update signal icon for -1 level in AddAppNetworksFragment

The level of a WifiEntry may be -1 (WIFI_LEVEL_UNREACHABLE) due to race
conditions. Thus, only update the signal icon if the level is a valid
value [0, 4]

Bug: 213477566
Test: m
Change-Id: I87f1c108d262bb5a7575c9ad434d2cfb953134af

src/com/android/settings/wifi/addappnetworks/AddAppNetworksFragment.java

@@ -414,6 +414,9 @@ public class AddAppNetworksFragment extends InstrumentedFragment implements
     }
 
     private void updateSingleNetworkSignalIcon(int level) {
+        if (level == WifiEntry.WIFI_LEVEL_UNREACHABLE) {
+            return;
+        }
         // TODO: Check level of the network to show signal icon.
         final Drawable wifiIcon = mActivity.getDrawable(
                 Utils.getWifiIconResource(level)).mutate();