unlockDialog: Use GdmUtil.AuthPrompt instead of ModalDialog

commit ea02380c made the login
screen stop using ModalDialog. It makes sense for the unlock
code to also stop using ModalDialog, too (for similar reasons).

Now that the login screen's auth prompt code has been separated
out, the unlock dialog can use it to get the buttons and spinners
etc, that it was previously getting from ModalDialog.

This commit drops the ModalDialog usage in the unlock dialog, and
makes the unlock dialog use GdmUtil.AuthPrompt instead.

https://bugzilla.gnome.org/show_bug.cgi?id=702308