Add UserSetupInteractor.

This exposes the isUserSetUp flow (renamed from isUserSetupFlow for
consistency with other repositories, and clarity) from
UserSetupRepository, for use in viewmodels or other non-domain layer
places.

Also took this opportunity to do some cleanup in associated code:
- renamed setup to setUp so it's more clear we're referring to the verb
- moved FakeMobileIconsInteractor to utils, since test dir should only
  contain tests
- moved UserSetupRepository to a more appropriate package, since it's
  similar to DeviceProvisioningRepository and will no longer only be
  used in the mobile/ package
- made UserSetupRepositoryTest multivalent, and updated how we check
  the flow (otherwise it was failing when running deviceless)

Bug: 293167744
Test: UserSetupInteractorTest
Flag: NONE
Change-Id: Idb4cc30266bfc15a63230e7e37afe60995701b69