[Sb refactor] Consume isInService in mobile view model

This implementation matches what was in MobileSignalController. If
isInService() reports false, then we show the empty mobile icon.
Otherwise use the regular icon id logic

Test: MobileIconViewModelTest
Test: MobileIconInteractorTest
Bug: 263167683
Bug: 238425913
Change-Id: Ie0da01439e79267a6842e096ec6924b3ccebce7d