Keyguard: do not allow to delete the system widget if widgets are disabled

We overwrite the default widget with our own one. In this case it is detected
as a normal widget even though it widgets are disabled and starts listening to
longpress events

Whatever check if widgets are disabled. If this is the case do not allow any
longpress events

Change-Id: I471664bac6ca1f8e57efe4904a385904c40486db