libui: Limit slider state to input devices that are actual keyboards

Change I6a6a18d0b09d0 ("modify the device to enable to get the
state as slide-open/slide-close.") caused issues on devices running
kernels that announced evdev switches for inputs other than keyboards
(headsets, for instance), which are open by default.

By generically treating an open switch as a keyboard presence
indicator, this change was causing devices with these switches to be
stuck in keyboard mode (i.e., landscape w/o rotation). So... limit
the new logic to alpha-keyboard inputs only

Change-Id: Id180598775288b9e60386c18d68d69e6fb90632d