Check biometrics availability when relevant

Check the availability of biometrics when we need to know, waiting until
then to decide the presence of ScreenLockActivity, rather than checking
once during initialization and permanently disabling pages of the setup
wizard. Otherwise, a race condition may cause the system to report that
the hardware is unavailable, even though it will be ready when needed.

Reorder pages such that biometrics comes first, allowing screen lock
to act as the fallback.

Issue: calyxos#2544
Change-Id: I654ce25569b983035f1df453623c7a4acd54931d