Merge pull request #20415 from ian-liu/bluetooth/bug1017506_unpair_devices_dialog_could_be_closed_by_clicking_on_places_other_than_the_buttons

Bug 1017506 - [Settings][Bluetooth] The un-pair devices dialog(modal) could be closed by clicking on places other than the buttons, r=arthur
(cherry picked from commit aa125bd4f448c65c94fca678ead21f6e68d239fc)