Refactor special handling of hidden API encoding for master-art

Instead of encoding the hidden API with an empty set of flags when the
monolithic flags are not available this simply disables encoding
altogether which should have the same behavior at runtime.

This change also removes the unused flags field in hiddenAPISingleton
which was set but never read.

Bug: 179354495
Test: m nothing
Change-Id: I32d5825e5271829993dd4e5be4d4ee1b22fa7b22