ScreenShield: lower the autodrag threshold to 10%

The threshold is there just to ensure that the user is opening
the curtain on purpose. Requiring 40% is too much for that.

https://bugzilla.gnome.org/show_bug.cgi?id=681143

js/ui/screenShield.js

@@ -20,7 +20,7 @@ const LOCK_ENABLED_KEY = 'lock-enabled';
 const CURTAIN_SLIDE_TIME = 1.2;
 // fraction of screen height the arrow must reach before completing
 // the slide up automatically
-const ARROW_DRAG_TRESHOLD = 0.4;
+const ARROW_DRAG_TRESHOLD = 0.1;
 
 const SUMMARY_ICON_SIZE = 48;