[Provider Model] Don't show Wi-Fi icon if the level is -1

- Since WIFI_LEVEL_UNREACHABLE (-1) is not 100% avoidable

- Don't show Wi-Fi icon if it's no level.

Bug: 198742993
Test: manual test
atest -c InternetDialogControllerTest

Change-Id: I9496d371a4ddfb71fd8e0c742c8e0d60a356a456