Keyguard: let the user drag the challenge down if widgets are disabled

for the system clock widget this does not make much sense but due
that our system default is DashClock it makes a lot sense to
allow it to see more content of DashClock extensions

So allow it for overlay added app widgets if the binding was
sucessfull.

Change-Id: I0365573c6fa88117a47b01ced81eb36f75324342