Show WiFi minimum level when the Wi-Fi level is UNREACHABLE

- Use WIFI_LEVEL_MIN instead of WIFI_LEVEL_UNREACHABLE to get the Wi-Fi icon resource to avoid app crashing because no valid Wi-Fi icon could be found.

Bug: 213294802
Test: manual test
make RunSettingsRoboTests ROBOTEST_FILTER=AddAppNetworksFragmentTest

Change-Id: Ia805e902716c25db3276b96116a84d0dd2d96a1b

tests/robotests/src/com/android/settings/wifi/addappnetworks/AddAppNetworksFragmentTest.java

@@ -485,6 +485,11 @@ public class AddAppNetworksFragmentTest {
         assertThat(item.mDisplayedSsid).isEqualTo(FAKE_NEW_WPA_SSID);
     }
 
+    @Test
+    public void getWifiIconResource_wifiLevelIsUnreachable_shouldNotCrash() {
+        AddAppNetworksFragment.getWifiIconResource(WifiEntry.WIFI_LEVEL_UNREACHABLE);
+    }
+
     private void setUpOneScannedNetworkWithScanedLevel4() {
         final ArrayList list = new ArrayList<>();
         list.add(mWifiEntry);