KeyGuard: minimize challenge when showing the lockscreen (2/2)

This will allow the user to choose wether the lockscreen
challenge (circle, pattern, pin) is minimized when the screen
is turned on.

Change-Id: I55c460df23d76d3fd5e620c8c2b46c83b05103b8
Signed-off-by: Dirk Rettschlag <dirk.rettschlag@gmail.com>
nope
Hide customize hardware keys checkbox
Hide the checkbox if the keymask is 0.