Changed ui fix

The UI changed, this is visible between master and v1-train that menu items are shifted a little bit so 
<code>menu_item.location['x'] == -305</code> and <code>menu_item.size['width'] == 290</code>
and changing the difference to <code>-15</code>

ALL the Menu Item tests are failing because of this:
http://qa-selenium.mv.mozilla.com:8080/view/B2G/job/b2g.unagi.mozril.gaia.master.ui/lastCompletedBuild/testReport/%28root%29/TestBluetoothSettings/test_toggle_bluetooth_settings/
http://qa-selenium.mv.mozilla.com:8080/view/B2G/job/b2g.unagi.mozril.gaia.master.ui/lastCompletedBuild/testReport/%28root%29/TestWallpaper/test_change_wallpaper/