[SB Refactor] Don't display the wifi icon if wifi isn't default.

Fixes: 272509965

Test: atest WifiViewModelTest

Test: on a device with no or invalid mobile connection, turn wifi off
then back on. Verify that you still see a wifi icon with the
exclamation mark first, then a wifi icon with no exclamation mark.

Test: on a device with a valid mobile connection, turn wifi off then
back on. Verify that you only see the wifi icon when the wifi is fully
connected (never see wifi icon with no exclamation mark)

Test: on a device with a valid mobile connection, enter an area with low
quality wifi. verify that you never see both the RAT icon ("LTE", "5G",
etc.) *and* the wifi icon at the same time.

Change-Id: I6693a03e8894c2e8eb8c08e528d6e53eae533d9b