Remove HiddenApiAccessFlags, move content to hiddenapi::

Hiddenapi code in runtime/ has all code in the hiddenapi:: namespace
instead of using a class as a wrapper. Refactor HiddenApiAccessFlags
for consistency. Also turn ApiList into `enum class` for stricter
type checks.

Test: m test-art
Change-Id: Ifb3c443ea43860476abd4fd3d4934cd14e2cdcc1