Improved user experience for drop down widgets on touch devices

Before this PR, when selecting an item in the popup of a drop down widget (such as a DatePicker, an AutoComplete, a SelectBox or a MultiSelect), the focus goes back to the input element.

On touch devices, focusing an input element triggers the display of the keyboard. This PR prevents the focus from going back to the input element immediately after selecting an element in the popup of a drop down widget on touch devices or when touching the dropdown icon.

The behavior is not changed on non-touch devices (the detection of touch devices is done through `aria.utils.Device.isTouch`).
I think there's been a wrong copy-paste here (comment speaks about _desktop_ platform).
Regarding that, I would (again, sorry) put the line below in a method dedicated to assert that the widget has focus. Easier to understand then when reading `checkFocusDesktop` and `checkFocusTouch`, and avoiding code duplication.