servicemanager: Fix UpdatableViaApex tests

These tests are supposed to run with known vendor apexes.
Make the skip conditions more explicit so that they are run on only
cuttlefish phones.

Todo: for better coverage, we could add more targets with known preinstalled
vendor apexes.

Bug: 280483521
Test: servicemanager_test
Change-Id: Id553583fa56aba586d588c90e263cab99188237e