Settings: Udfps: Use same ambient config logic as others

Others like DoubleTapScreenPreferenceController or
PickupGesturePreferenceController use getAmbientConfig() in
getAvailabilityStatus - looks like they experienced issues
with using the config set via setConfig() as well

Fixes: Attempt to invoke virtual method
'java.lang.String android.hardware.display.AmbientDisplayConfiguration.udfpsLongPressSensorType()'
on a null object reference

Change-Id: I68c94bd295e324cc8668dd06b8178876787f1dc6