Add an option to not send the lock password in the SMS lock message.

I see you've changed things up in the current source a bit since the last released version on Google Play and that you've added an option to turn off the confirmation SMS completely but I'd still like to receive the confirmation, I just don't want it to contain the password. You don't have to merge this pull, just thought I'd create a pull request in case it was found useful. Also, I haven't done any Android development so beware, I don't really know what I'm doing. :-)

When the app receives an SMS to lock the device it sends a SMS response
informing the sender the device was locked. The password used to lock
the device is sent along with the confirmation. A checkbox now exists in
the lock settings to enable/disable the sending of the password in the
SMS response.