fs_mgr: use __ANDROID_API_Q__ instead of pre_gki_level

The name "pre_gki_level" is causing some confusion because not all
devices launching with Android R are subject to the GKI requirement.
(See b/161563110#comment11.)  E.g., devices that use a 4.14-based kernel
are exempt from GKI.  However, the encryption requirements still apply.

Just use __ANDROID_API_Q__ directly instead.

No change in behavior.

Change-Id: I4242745ccc9f88e084a1adfab5796daa9bc59b6e