Get rid of public reference to KeyboardIconsSet.ICON_* constants

As a consequence,
  * Add Key.iconDisabledKey has been introduced and Key.getIcon honors
    the enabled state of the key.
  * The attribute id of disabled icon for shortcut key,
    Keyboard_iconShortcutKeyDisabled, is renamed to
    Keyboard_iconDisbledShortcutKey
  * KeyboardIconsSet has getIconByIconId and getIconByAttrId methods
    instead of getIcon.

Bug: 5778201
Change-Id: Ica93b073b9a04acd18ead7a33b60e3c6d813e7a1