Fix keyboard background with HideFormAccessoryBar

Fix CB-6219 by preventing the main keyboard from becoming transparent on iOS 7. (This looked particularly bad on iOS 7.1, where a white background would render the Shift key icons invisible.)

Thanks to http://stackoverflow.com/questions/18837551/remove-keyboard-form-toolbar-on-ios7-leaves-a-blur-behind/19042392#comment28210345_19042392