[SB Refactor] In the mobile view model, move `isInService` calculation.

While looking into b/266625531, I almost missed the fact that
`isInService` is used to determine whether or not to show the ! because
the `isInService` wasn't part of the `showExclamationMark` flow. And,
that means that our `showExclamationMark` logs might be wrong, because
`isInService=false` will cause us to show the ! but our logs would say
`showExclamationMark=false` incorrectly.

This CL:
1) Moves `isInService` to be part of the `showExclamationMark` flow
   instead of the `icon` flow.
2) Creates a new `shownLevel` flow, which then simplifies the `icon` and
   `contentDescription` flows.

Bug: 238425913
Test: atest MobileIconViewModelTest (existing tests already cover
`isInService` affecting ! and level)

Change-Id: Ibfc64cac91e1df537cc2b44635232cd0f848be56