Remove feature flag for active unlock

Active Unlock was guarded behind a feature flag for two main purposes.
1) To select the unlock intent or biometric failure layout. We've
   settled on unlock intent, so we no longer need the biometric failure
   codepath.
2) To guard the feature rollout. However, the additional platform guard
   is not necessary, since this feature availability is still guarded
   based on whether the secure settings are flipped to valid targets.
In order to keep the scope of this CL small, useBiometricFailureLayout
always returns false. Another CL will clean up all areas where it and
useUnlockIntentLayout are referenced.

Test: make RunSettingsRoboTests
Bug: b/280306741
Change-Id: I331afe9654b72249d461007c087e7f81783d7625