OTA: Make the final ConfirmBox undismissable

It's relatively easy to miss-click the buttons, especially on devices
with questionable touch panels...

And while doing so is perfectly fine, you miss the InfoMessage telling
you that the update will be applied on restart, which is potentially
confusing.

Re: #12674

frontend/device/generic/device.lua

@@ -455,9 +455,11 @@ function Device:install()
             UIManager:show(InfoMessage:new{
                 text = _("The update will be applied the next time KOReader is started."),
                 unmovable = true,
+                dismissable = false,
             })
         end,
         unmovable = true,
+        dismissable = false,
     })
 end