servicemanager: expose updatable-via-apex

Service manager is already reading VINTF, so processes that need this
information can access this information w/o needing more permissions.

Bug: 185832616
Test: two added tests + manually specifying updatable-via-apex on a
    service and checking the result
Change-Id: Id86a72d49466048a3e2173e40526981abb6f3123