Don't always set alternateBouncerUIAvailable to true

When the DeviceEntryUdfpsRefactor is enabled. Instead, this is
handled in the AlternateBouncerInteractor.

Test: device builds, boots and shows keyguard with the
device entry udfps refactor flag enabled
Flag: ACONFIG com.android.systemui.device_entry_udfps_refactor DEVELOPMENT
Bug: 287599719

Change-Id: I32cb6da78472b394bf7f33dd04a74d140621d514